Garden Glossary

AGM

AGM stands for Award of Garden Merit, an award the RHS gives to plants that have performed reliably well in trials and general cultivation across the UK. It's a genuinely useful signal of a plant's all-round garden performance, hardiness and availability, but it's a UK-wide award, not a Scottish one, and is never treated on this site as Scotland-specific growing advice.

Backfill

Backfill is the soil used to fill in around a plant's roots or rootball once it's been placed in its planting hole, firmed in to remove large air pockets and give the roots solid contact with the ground. Instructions to mix grit or compost into the backfill mean improving that fill-in soil specifically, not the whole bed.

Bare-root

A bare-root plant, most often a fruit tree, soft fruit bush or hedging plant, is lifted from the nursery field with its roots exposed rather than grown in a pot of compost. It's cheaper than the same plant container-grown, and often establishes just as well or better, but it's only available and only safe to plant during the dormant season, roughly November to March, and needs planting promptly rather than sitting around.

Biennial

A biennial plant grows leaves and roots in its first year, then overwinters and, in its second year, sends up a flowering stem, sets seed and dies. Parsley is the most familiar biennial vegetable: it's grown and harvested as if it were an annual, for its leaves in year one, but if left in the ground will flower and set seed the following year. Contrast with an annual (completes its life cycle in a single year) and a perennial (lives and can flower repeatedly for several years or more).

Biennial bearing

Biennial bearing is a fruit tree's tendency to crop heavily one year, using up most of its stored energy in the process, then crop lightly or barely at all the following year while it recovers, and repeat that heavy-light cycle indefinitely rather than producing a moderate, even crop every year. Some varieties are simply more prone to it than others regardless of how they're managed; thinning a heavy year's fruit early, before the tree commits fully to it, is the main way to even the cycle out.

Blanching

Blanching keeps light off part of a plant while it's still growing, by earthing up, wrapping in card or fabric, or covering with an upturned pot, so that part develops without producing the chlorophyll that turns tissue green, bitter and tough. It's used on leeks (the earthed-up white shaft), celery, endive and cardoons, and is different from the kitchen technique of the same name, which briefly boils vegetables after harvest.

Bolting Full page →

Bolting is a plant switching prematurely from growth into flowering and seed production, usually triggered by a stress signal such as a cold snap, a heatwave, drought, or simply day length for some crops. Once bolted, leafy crops usually turn bitter and roots or bulbs turn woody or split, and for most vegetables the crop is lost rather than recovered. It affects different crops for different reasons and at different points in the season.

Brassica

Brassica is the plant family that includes cabbage, kale, broccoli, cauliflower, Brussels sprouts, kohlrabi, swede, turnip, radish and rocket among others. The grouping matters practically, not just botanically: every brassica is vulnerable to the same core problems, clubroot, cabbage root fly, flea beetle, whitefly and caterpillars among them, and shares the same rotation rule of not following another brassica in the same ground for several years, since pests and clubroot spores that target the family persist in soil regardless of which individual brassica grew there last.

Catch crop

A catch crop is a fast-maturing crop, radish and salad leaves are typical choices, sown into a short-term gap: the weeks between clearing one main crop and planting the next, or the space at the edge of a bed a slower crop hasn't filled yet. It squeezes an extra harvest out of ground and time that would otherwise go unused.

Central leader / half-standard

Central leader and half-standard are two more fruit tree training shapes, alongside cordon, espalier and fan. A central-leader tree keeps one dominant, upright main trunk with side branches coming off it in tiers, closer to a traditional pyramid-ish shape than an open-centred bush. A half-standard is a free-standing tree grown with a clear trunk of roughly 1.2–1.5m before branching starts, tall enough to walk or mow underneath, midway between a compact bush tree and a full standard's taller clear trunk.

Chitting

Chitting means standing seed potatoes eye-end up, somewhere light and cool but frost-free, for four to six weeks before planting. It encourages short, dark green, sturdy sprouts rather than the long pale ones a warm, dark spot produces, and gives the plant a head start once it's in the ground. It matters most for first earlies, where the extra weeks make the difference between a June and a July harvest; maincrop potatoes, in the ground for longer regardless, benefit less.

Cloche

A cloche is a small, portable, clear cover, glass, rigid plastic or a plastic tunnel over hoops, placed directly over an individual plant or a short row in open ground. It traps warmth and shelters the plant from wind, heavy rain and some pests, letting a sowing go out earlier in spring or extending a harvest later into autumn than the same crop, uncovered, could manage. It's smaller-scale and more temporary than a cold frame or polytunnel, but does a similar job.

Clubroot

Clubroot is a soil-borne fungal disease specific to the brassica family, cabbage, kale, turnip and their relatives, that infects roots and causes them to swell and distort into club-like galls, badly stunting growth above ground. Once present, its resting spores can survive in soil for many years, so infected ground needs a long rotation break from brassicas, plus liming to raise soil pH, which slows the fungus without eliminating it.

Containerised

A containerised plant is grown and sold in a pot of compost rather than lifted from open ground, which means it can be planted out at any time of year the ground isn't frozen or waterlogged, not just during the dormant season a bare-root plant needs. It generally costs more than the same plant bare-root, since more time, compost and pot space go into producing it, but it gives far more flexibility about when to plant.

Cordon

A cordon is a fruit tree or bush pruned and trained into a single stem (or occasionally two or three), with side growth kept cut back to short fruiting spurs rather than allowed to branch freely. Grown upright or at a slant against a wall, fence or wire framework, cordons fit far more trees, and more varieties, into a small garden than free-standing trees would allow. Apples and pears are the most common cordon-trained fruit; tomatoes grown as a single stem up a cane or string are sometimes called cordon-trained too.

Crop rotation

Crop rotation means not growing the same crop, or crop family, on the same patch of ground in consecutive years, instead moving each family around a plot on a multi-year cycle (commonly three or four years). Many soil-borne pests and diseases, clubroot and white rot among them, and some nutrient depletion patterns, build up specifically around the crop family that hosts them; moving that family elsewhere for several years starves the problem out rather than letting it re-infect the same ground every season.

Cross-pollination

Cross-pollination is pollen moving from the flowers of one plant to a different, compatible variety, usually carried by insects, rather than a flower pollinating itself. Any fruit variety that isn't self-fertile depends on cross-pollination from a suitable pollination partner nearby to set a worthwhile crop; without it, flowers can open and still fail to develop into fruit.

Crowns

A crown is the base of a perennial plant, the point where roots below and shoots above meet, and the part from which new growth emerges each year. Asparagus, rhubarb and strawberries are commonly sold and planted as bare-root crowns rather than raised from seed, giving a faster-cropping, more predictable plant than a seed-grown one of the same age.

Cultivar

A cultivar (a blend of "cultivated variety") is a plant that was selected or bred for a particular trait, colour, flavour or habit, and is kept true to that trait only because growers deliberately propagate it, by seed that breeds true, cuttings, grafting or division, rather than it persisting on its own in the wild. "Cultivar" and "variety" are used interchangeably in everyday gardening language, including on this site; the more technical distinction (a botanical variety occurs in the wild, a cultivar exists because of cultivation) rarely matters for a home gardener.

Curd

The curd is the edible part of a cauliflower: a tightly packed head of undeveloped flower buds, harvested before it opens out and starts to flower. A good curd is firm and compact; irregular watering, stress or leaving it too long past ready all tend to loosen or discolour it, which is why timing the cut matters more on cauliflower than on most vegetables.

Damping off

Damping off is a fungal disease that attacks seedlings at or just below the soil surface, causing the stem to blacken, thin and collapse, often wiping out a whole tray within days. It's driven by conditions that suit the fungus: overwatering, poor airflow, overcrowded sowing, and reused pots or trays that weren't cleaned first. Once a seedling has collapsed it can't be saved; prevention (clean equipment, careful watering, good ventilation) is the only real control.

Deadheading

Deadheading is the routine removal of flowers once they've faded, cut or pinched off before they can set seed. Because producing seed takes considerable energy, removing the spent flower redirects that energy back into further flowering (on ornamentals) or into growth and cropping (on some edible crops), rather than the plant winding down once it's set its first batch of seed.

Determinate / indeterminate

Determinate and indeterminate describe a tomato variety's growth habit. Determinate, or bush, varieties stop growing at a genetically set height, need little or no side-shooting, and ripen most of their fruit in a shorter window. Indeterminate, or cordon/vine, varieties keep producing new growth and new trusses until something stops them, cold weather, or the grower pinching out the growing tip, and need regular side-shooting and support to stay manageable.

Dibber

A dibber is a simple, pointed hand tool, sometimes just a piece of wood shaped to a point, pushed into prepared soil to make a neat planting hole of a consistent depth for a seedling, set or young plant, which is then dropped in and firmed around. It's a low-tech alternative to digging each hole with a trowel, and is quicker for planting out a row of small plants in one go.

Division

Division propagates a clump-forming perennial, rhubarb and many herbs among them, by lifting the whole plant and splitting it into several smaller sections, each with a share of both roots and shoots, then replanting the pieces separately. It's both a way to make new plants for free and a way to rejuvenate an old clump that's become overcrowded or has stopped cropping well in the middle.

Drill

A drill, in a sowing context, is a shallow, straight groove made in prepared, raked soil, usually with a cane, hoe corner or trowel edge, that seed is sown along at a consistent depth. Sowing into a drill rather than scattering seed keeps rows straight and depth even, which makes germination more uniform and later thinning or weeding much easier to do accurately.

Dwarfing / vigorous (rootstock)

Dwarfing, semi-dwarfing, semi-vigorous and vigorous describe how much a fruit tree's rootstock (see rootstock) restricts or allows growth, on a scale running roughly from extremely dwarfing (a tree that might reach 1.5–2m, suited to a pot or tiny garden) through to very vigorous (a tree that can exceed 6–8m, closer to a traditional orchard tree). The same fruiting variety grafted onto rootstocks at different points on this scale can end up very different final sizes, which is the main thing to check before buying a tree for a small garden.

Earthing up

Earthing up means drawing soil (or compost) up into a ridge around a plant's stem as it grows, done in stages as the foliage gets taller. On potatoes it's essential, not optional: light reaching a developing tuber turns it green, and green potato flesh is toxic. It also buries and protects the lowest, most vulnerable part of the stem and gives extra support to a plant that's putting on height fast.

Espalier

Espalier training produces a fruit tree with a single vertical trunk and pairs of horizontal branches trained flat along wires at evenly spaced tiers, usually against a wall or fence. It's both decorative and space-efficient, turning a boundary into a productive fruit wall, and is most commonly used for apples and pears, which fruit well on the short spurs this shape encourages.

F1 hybrid

An F1 hybrid is the first-generation offspring of two specific, genetically distinct parent plants, deliberately cross-bred to combine particular traits, disease resistance, uniformity, vigour, from each parent. F1 varieties are typically more uniform and predictable than open-pollinated ones, but seed saved from an F1 plant won't grow true to type the following year, since the next generation's genetics reshuffle rather than repeat the parent cross, so F1 seed is bought fresh each time rather than saved.

Fan-trained

Fan training spreads a tree's branches out flat like a hand fan from a low point near the base, tied onto a wall or wire framework. It's the standard training shape for plums, cherries, apricots and other stone fruit, which don't respond well to the rigid tiers of an espalier, and it's also used for figs and some soft fruit against a sheltered wall.

Fleece

Horticultural fleece is a lightweight, breathable, spun fabric laid loosely over plants or pegged over hoops, letting light, air and rain through while trapping enough warmth underneath to give a few degrees of frost protection and take the edge off wind. It's also used as a physical barrier against flying pests, carrot fly and cabbage white butterflies among them, when draped over a crop from the moment it's sown or planted so the pest never gets access to lay eggs on it.

Florets

Florets are the individual small clusters of unopened flower buds that together make up a broccoli, cauliflower or romanesco head, whether the tight central curd/head or the smaller side-shoots some crops produce after the main head is cut.

Flowering group / pollination group

Flowering group, or pollination group, classifies fruit tree varieties, apples and pears especially, by roughly when they come into blossom each spring. Two varieties in the same or adjacent flowering groups overlap enough for cross-pollination to work; varieties in distant groups may simply never flower at the same time, which matters when choosing a pollination partner for a variety that isn't self-fertile.

Foliar feed

A foliar feed is a liquid fertiliser sprayed directly onto a plant's leaves, where it's absorbed faster than a feed applied to the soil and taken up by the roots. It's typically used as a quick response to a specific, visible deficiency, or as a supplementary boost during heavy cropping, rather than as the main feeding routine for a plant.

Forcing

Forcing covers a dormant crown with a pot, bucket or purpose-made forcing pot in late winter, excluding light entirely to trick it into early, fast, pale and unusually tender growth. Rhubarb is the classic example, forced for the earliest, sweetest stems of the year, and sea kale is traditionally forced the same way for blanched, tender shoots. A forced crown needs a season or two to recover afterwards before it can be forced again.

Free-draining

Free-draining soil lets water pass through and downward relatively quickly after rain, rather than pooling at the surface or staying saturated at root level, which most plants need to avoid waterlogging and root rot. Sandy or gritty soils tend to be free-draining naturally; heavy clay usually isn't, unless it's improved with grit or organic matter, or the plant is grown on a raised mound or ridge instead.

Frost pocket

A frost pocket forms where cold, dense air drains downhill on a still, clear night and collects in a low spot, a dip, the foot of a slope, or anywhere hemmed in by a wall, hedge or fence that blocks it from draining further. That spot can see a noticeably harder, more frequent frost than ground only a short distance away at the same regional altitude, which matters when deciding where to site frost-tender crops.

Germination

Germination is the process of a seed breaking dormancy and beginning to grow, putting out its first root and shoot, triggered by the right combination of moisture, warmth and, for some seeds, light or a period of cold. Germination time varies hugely by species, from a few days for fast crops to a month or more for seeds with a hard coat or another dormancy mechanism to overcome, such as parsley or angelica.

Graft

Grafting joins two plants together so they grow and function as one: a cutting of the desired fruiting variety, the scion, is joined onto a separate rootstock, which supplies the roots and controls the tree's eventual size. Almost every fruit tree sold, apples and pears especially, is grafted rather than grown from seed, because fruiting varieties don't come true from seed and a named variety can only be reliably reproduced by propagating a cutting from an existing tree of that exact variety.

Green manure

A green manure is a crop, often a legume like field beans or clover, or a fast grower like mustard, sown not to be harvested but to be cut and dug back into the soil (or hoed off and left to rot on the surface) while still green. Legume green manures fix nitrogen from the air into the soil as they grow; all green manures add organic matter as they break down, and can also outcompete weeds and reduce nutrient leaching from bare winter ground.

Growth habit

Growth habit describes a plant's characteristic shape and manner of growing: upright, spreading, creeping, trailing, bushy or compact, for example. It's used to compare varieties of the same plant that grow differently, an upright rosemary versus a prostrate one, or a creeping comfrey versus a clump-forming one, and matters for how much space a plant needs and how it's best used in a garden.

Hardening off

Hardening off is the step between raising a plant indoors or under cover and planting it out in open ground. Move it outside for a few hours a day, somewhere sheltered at first, and extend the time over one to two weeks, bringing it back in or under cover on the coldest nights, until it can sit outdoors full-time without wilting, scorching or being knocked back by wind. Skipping it, or rushing it, is one of the most common reasons a batch of seedlings that looked fine indoors collapses within days of planting out.

Hardiness rating

The RHS hardiness rating is a scale from H1a (needs a heated glasshouse, can't survive outdoors at all) through H1b, H1c and H2 (increasing tolerance of cool but frost-free conditions) up to H3–H7, which cover outdoor hardiness from tender-needs-shelter (H3) to hardy in the coldest UK winters (H7). A named rating on a variety, such as H5 (hardy to around -15°C) or H6 (hardy to around -20°C), gives a concrete, comparable figure for how much winter cold that specific variety is expected to survive outdoors unprotected.

June drop

June drop is the natural shedding of some immature, small fruit that apple and pear trees do in early summer, the tree's own way of thinning a crop it can't fully support to maturity. It's a normal, healthy process rather than a sign of a pest, disease or watering problem, and it often reduces or removes the need for a gardener to thin the remaining fruit by hand afterwards.

Layering

Layering propagates a new plant from a low, flexible stem that's bent down and pinned or buried into the soil while it's still attached to, and fed by, the parent plant. Once the buried section has rooted on its own, usually after some months, it's severed from the parent and becomes an independent plant. It's a common, low-effort way to propagate many soft fruit and shrubs, including blackberries and other cane fruit that layer naturally where a cane tip touches the ground.

Leggy

A leggy plant has grown tall and thin, with long bare gaps between leaves or side shoots rather than a compact, bushy shape. It's usually caused by too little light, seedlings stretching towards a distant window, or a plant that hasn't been pinched out to encourage side growth. A leggy plant is often weaker and more prone to flopping over than a stockier one, and pinching out or increasing light are the usual fixes.

Maincrop

Maincrop describes the main type or sowing of a crop, typically the latest-planted and longest in the ground of that crop's types, grown for bulk yield and storage life rather than speed. Maincrop potatoes and maincrop onions are the most common uses of the term: both are planted later than their "early" counterparts, take longer to mature, and are the type gardeners rely on for a winter store rather than the earliest possible harvest.

Microclimate

A microclimate is the set of growing conditions particular to one small, specific spot, a sheltered walled corner, a south-facing slope, the sunny side of a shed, that can differ substantially from the average conditions of the wider garden or region. A warm microclimate can let a marginal crop succeed where the regional climate alone would suggest it shouldn't, and identifying the microclimates within a single garden is often more useful than any general regional advice.

Modules

A modular tray, or module tray, is divided into small individual cells rather than one open compartment, with each cell (module) sown with one or a few seeds. Growing in modules means each seedling develops its own contained, undisturbed root system, and can be planted out, or "potted on," with minimal root disturbance compared with pricking out a seedling from an open seed tray, which is why many crops that dislike root disturbance are specifically recommended for module-sowing.

Mulch

A mulch is a layer of material, most often garden compost, well-rotted manure, bark or straw, spread over bare soil around plants. It suppresses weeds by blocking light, slows moisture loss from the soil surface, insulates roots against temperature swings, and, for organic mulches, gradually improves soil structure as worms and soil life work it in.

No-dig

No-dig growing avoids turning the soil over with a spade or fork, adding compost and other organic matter as a surface mulch instead and leaving worms and soil life to work it down. The aim is to preserve the soil's existing structure, drainage channels and fungal networks, which digging disrupts, while still building fertility over time. It's a method, not a single fixed technique, and how strictly it's applied varies grower to grower.

Old wood / young wood

Old wood and young (or new) wood distinguish a plant's older, established growth from stems it made more recently, usually in the past year, and the distinction decides how and when it gets pruned. A plant that fruits on young wood, blackcurrants are the standard example, needs a portion of its older wood removed each year to keep making the young growth that will fruit next; a plant that fruits on old wood or established spurs needs that framework preserved rather than cut away.

Open-centred (goblet-shaped)

An open-centred, or goblet-shaped, bush is pruned to keep the middle open, with main branches radiating outward and upward from a short leg rather than growing inward and crossing. It's the standard training shape for gooseberries, redcurrants and whitecurrants: an open centre lets light and air reach every branch, which improves fruiting and airflow (helping keep mildew down) compared with a dense, congested bush shaded at its core.

Overwinter

Overwintering means keeping a plant, whether it's growing, dormant, or a stored root or tuber, alive and intact through the cold months, until conditions are right for it to grow again or be planted out in spring. Depending on the plant, that might mean leaving it outdoors as-is, giving it a protective mulch, moving a pot under cover, or lifting and storing a tender plant somewhere frost-free entirely.

Peat-free

Peat-free compost is made from alternative materials, coir, bark, wood fibre, composted green waste and similar, instead of peat, which is partially decomposed plant matter dug from bogs. Peat bogs take centuries to form, store large amounts of carbon, and support habitat that peat extraction damages, which is why peat-free compost is now the standard recommendation and, for most retail bags, close to the only option available.

Perpetual / everbearer

Perpetual, or everbearer, strawberry varieties produce a series of smaller flushes of fruit spread from early summer through to autumn, rather than the single, larger, concentrated crop a standard summer-fruiting variety produces over two or three weeks. They give a steadier trickle of fruit over a longer season, at the cost of never producing quite the same glut a summer-fruiting variety does in its short peak window.

Pinching out

Pinching out removes the soft growing tip of a stem, usually just with finger and thumb, which stops that stem's upward growth and triggers side shoots to develop instead, producing a bushier plant. It's used on tomatoes (removing side shoots, or stopping the main stem once enough trusses have set), basil and other herbs (to stop plants going leggy and encourage bushiness), and broad beans (removing the growing tip once flowering starts, which also helps deter blackfly).

Pollination partner

A pollination partner is a different, compatible variety of the same fruit, flowering at a similar time, planted close enough for pollinating insects to move between the two. Any variety that isn't self-fertile needs one to set a worthwhile crop; without a partner in range, flowers can open, look fine and still fail to produce fruit.

Potting on

Potting on moves a plant from its current pot into a larger one, usually the next size up rather than a big jump, once roots have filled the existing pot and started circling its edges. It gives the roots fresh compost and room to keep developing, and delays the point at which a plant becomes pot-bound, stops growing well and needs watering constantly.

Pricking out

Pricking out means lifting individual seedlings, handled gently by a leaf rather than the stem, out of the crowded seed tray or pot they germinated in and moving each one into its own space, once it has grown its first true leaves (the second set to appear, after the initial seed leaves). It gives roots room to develop without competition before the seedling is potted on or planted out.

Puddling in

Puddling in means filling a planting hole with water, either before dropping the plant in or straight after, and letting it soak away rather than watering lightly from above. The water settles loose soil tightly around the roots as it drains, closing air pockets that would otherwise dry roots out, and it's used especially for leeks, where it also helps wash soil down into the dibbed hole around the stem.

Rootball

The rootball is the whole mass of a plant's roots together with the soil or compost bound up in them, as a single unit, whether it's tipped out of a pot or lifted from open ground. Planting instructions that reference the rootball, digging a hole wider than it, teasing circling roots out at its edges, are describing that whole root-and-soil mass rather than the roots alone.

Rootstock

Fruiting varieties don't come true from seed, so fruit trees are propagated by grafting a cutting of the desired variety onto a separate rootstock, which supplies the roots and lower trunk. The rootstock is chosen for what it does to the tree's growth, most importantly its eventual size (dwarfing, semi-vigorous, or vigorous), and sometimes disease resistance, not for any fruit of its own. The same apple variety on a dwarfing rootstock might reach 2–3m; on a vigorous one, well over 6m.

Runners

Runners are long, thin stems a plant sends out along the soil surface, rooting and forming a new, genetically identical plant wherever a node touches ground, still connected to and fed by the parent until it's established. Strawberries are the classic example, producing runners freely through summer that can be pegged down, rooted and severed to make new plants for free; mint spreads by underground runners (technically rhizomes) in much the same way, which is why it's usually grown contained.

Self-fertile

A self-fertile fruit variety can pollinate itself and set a full crop on its own, which matters most in a small garden with room for only one tree or bush. The opposite is self-sterile, or needing a pollination partner: a different, compatible variety flowering at the same time, planted close enough for insects to move pollen between the two, without which the plant sets little or no fruit at all.

Sets

Sets are small, immature bulbs, most commonly onion or shallot, that are planted directly into the ground rather than raised from seed. They give a faster, more reliable crop than seed-grown alternatives, and are more forgiving of a short or unpredictable growing season, at the cost of a smaller choice of varieties than seed usually offers.

Spur-bearer / tip-bearer

Most apples and pears are spur-bearers: they fruit mainly on spurs, short, stubby side-shoots that form along two- and three-year-old wood and go on cropping for several years, which is what pruning to "encourage fruiting spurs" is aiming to build. A minority of varieties, and quince entirely, are tip-bearers, fruiting mainly at the tips of the previous summer's new growth instead. The distinction matters for pruning: cutting back a tip-bearer's new growth hard, the way you would to encourage a spur-bearer, removes the very growth that would have fruited.

Successional sowing

Successional sowing spreads a crop's sowing dates out, a short row or small batch every two to three weeks through the sowing season, rather than one large sowing all at once. The result is a steady, staggered supply as each batch matures a few weeks after the last, avoiding both the glut of a single big harvest and the gap that follows it. It works best on fast-maturing crops that can fit several sowings into a season, salad leaves, radish and beetroot among them.

Sucker

A sucker is a shoot that grows up from a plant's roots or the base of its trunk, distinct from the main branch framework above. On grafted fruit trees a sucker grows from the rootstock rather than the fruiting variety, and left in place will eventually outcompete the graft; on raspberries and some other cane fruit, suckering is the plant's normal way of spreading and is only a problem where it's spreading further than wanted. Suckers are usually pulled or cut off close to their point of origin.

Tap root

A tap root is a single, thick, dominant root that grows straight downward from a plant, often swollen to store energy (carrots and parsnips are essentially an edible tap root), as opposed to a fibrous root system made up of many fine, branching roots of similar size. Plants with a tap root, parsley and dill among them, generally dislike being transplanted once the tap root has formed, since it's easily damaged and doesn't recover or branch out the way fine roots can.

Tilth

Tilth describes the physical texture of prepared soil: raked or hoed to a fine, crumbly consistency with no large clods, rather than the coarser structure left after digging. A fine tilth is what most small seeds are sown into, since large lumps of soil leave gaps that stop tiny seeds from making the even contact with moist soil they need to germinate.

Top-dressing

Top-dressing adds a layer of fresh compost, a slow-release feed, or grit to the soil surface around an established plant, without disturbing its roots by digging. It's the usual way to refresh nutrients for a permanent or perennial planting, containers included, where full replanting or repotting isn't practical or necessary every year.

Triploid

A triploid variety carries three sets of chromosomes instead of the usual two, which generally makes its own pollen poor at fertilising other trees, even though the triploid tree itself can usually be pollinated normally by a suitable diploid variety. Practically, this means a triploid needs two other, compatible varieties nearby, one to pollinate the triploid, and a second to pollinate each other, since the triploid can't reliably do that job itself.

Truss

A truss is the branching stem on a tomato plant that carries a cluster of flowers, and later a cluster of fruit, at one point along the main stem. A plant produces several trusses up its length as it grows, each ripening its fruit at roughly the same time, which is why a healthy indeterminate plant is picked in staggered batches, truss by truss, rather than all at once.

Tuber

A tuber is a swollen underground stem or root that a plant uses to store energy over winter or a dormant period, and which can also sprout into a new plant from buds ("eyes") on its surface. The potato is the classic example, a swollen underground stem rather than a root; Jerusalem artichoke also grows tubers. Tubers are usually planted directly, rather than grown from seed, to reproduce a specific variety reliably.

Weed-suppressing membrane

Weed-suppressing membrane is a permeable woven or spun fabric laid over bare soil, then cut with individual planting holes, that blocks light to weed seedlings underneath while still letting water and air pass through to plant roots. It's commonly used under strawberries and other low-growing fruit to keep weeds down and stop developing fruit sitting directly on wet soil, without resorting to a sealed plastic sheet that would also block water.