Top 5 Crops for Heavy Clay Soil

If your garden turns from cracked plates in a dry August to sticky mud by November, you have probably been told to dig in grit and compost. That is good advice, but improving heavy clay properly can take seasons. This page is about what you can plant in the clay you have now, unimproved: comfrey, damsons, water mint, lovage and quince. Most vegetables did not make the list because they want free drainageSoil that lets water pass through quickly rather than pooling, so roots aren't sitting in wet ground after rain. Full definition →.

What "suits heavy clay" means

Heavy, unamended clay gives a plant two problems. It compacts hard, so shallow or delicate roots struggle to establish, and it holds water so tightly that roots can waterlog over winter, then dries and cracks in summer. BrassicasThe cabbage family: cabbage, kale, broccoli, cauliflower, Brussels sprouts, kohlrabi, swede, turnip and more, sharing the same pests, diseases and rotation rules. Full definition →, beans and potatoes turn up on many clay lists, but drainage tolerance is what matters on ground that stays wet all winter. Cabbages want a sunny site with fertile, free-draining soil and broad beans rich, free-draining soil (RHS), and brassicas carry a risk of clubroot, which acidic soil favours and poor drainage makes worse. The five below each have a recorded tolerance of clay or consistently wet ground.

Comfrey

Comfrey is not a food crop: it is a cut-and-come-again source of mulchA layer of material spread over the soil around plants to suppress weeds, retain moisture, and improve soil structure as it breaks down. Full definition → and liquid feed. It suits clay and loam, and it grows in sun or partial shade. It is rated H7The RHS's H1a–H7 scale for how much cold a plant can survive outdoors, H1a being tender indoor-only and H7 hardy anywhere in the UK. Full definition →, hardy below −20°C. Common comfrey (Symphytum officinale) grows to 1 to 1.5m and is listed for moist but well-drained or poorly-drained soil. The cultivarA plant variety that arose or was bred under cultivation and is kept true by deliberate propagation, rather than a naturally occurring wild species. Full definition → usually planted for liquid feed, 'Bocking 14', is sterile, so it does not self-seed. It reaches 1.5 to 2.5m tall and 0.5 to 1m wide, and prefers moist but well-drained soil, so on clay give it a damp spot rather than one that stands in water. See the best comfrey varieties for Scotland.

Comfrey is harmful if eaten repeatedly, so it is not a foraged salad leaf, and it contains pyrrolizidine alkaloids that can damage the liver, so keep it away from pets, rabbits especially. Wear gloves to cut it, and cut plants back to the base to harvest the leaves. Plant in spring or autumn and allow generous space: a clump spreads to a metre or two, goes on producing for years, and is hard to remove once established.

Damsons

Damsons do not like to dry out, especially while the fruit is forming, so they generally prefer a loamy or clay soil, as long as it does not get waterlogged. Many varieties are rated H6, hardy to between −20 and −15°C, and damsons are generally hardier than plums. The blossom is frost sensitive, though, so choose a warm, sheltered site in full sun and avoid frost pocketsA low-lying spot where cold air drains and pools on still nights, making it noticeably more frost-prone than the surrounding area. Full definition → and strong winds, which can damage the blossom and deter pollinators. In colder sites, train damsons against a south-, south-west or west-facing wall or fence.

The site must not be waterlogged: clay as a soil texture is fine, clay that sits wet all winter is not. Plant on the highest, best-drained spot you have, or on a slightly raised mound. The widely sold damson varieties are self-fertileA plant that can set fruit using its own pollen, without needing a second, different variety nearby to cross-pollinate it. Full definition →, so one tree is enough, but check before buying. Plant a bare-rootA plant, usually a fruit tree or hedging, sold with its roots exposed rather than in a pot, and only available while dormant. Full definition → tree between November and March, while it is dormant. See the damson growing guide.

Water mint

Culinary mint thrives in sun or partial shade in a range of soil types, but water mint is the wetland species: clay and loam are listed as suitable, with poorly-drained moisture, and it is a plant of bogs and margins, native to Britain and Ireland. It grows in full sun or partial shade, reaches about 90cm and is rated H6. That makes a permanently damp corner of heavy clay a natural home for it. Its flavour is stronger and more medicinal than garden mint, so site it in the wettest patch rather than a general herb bed and use it sparingly, for tea or in place of mint sauce.

Like other vigorous mints, it spreads by rhizomes and is best grown in a pot or other container to keep it contained: a bottomless bucket sunk into the ground works. See the mint growing guide.

Lovage

Lovage is an exception to the rule that herbs want sharp drainage. Most soils are suitable, but it grows best in rich, moist soil that does not dry out or get waterlogged, and clay is listed among its soils, alongside loam, chalk and sand. It is a hardy perennial rated H6, related to celery and parsley, with celery-scented leaves, and flower stalks reaching 2m by mid to late summer. Plant in late spring or early summer in light shade if you have it, though full sun is fine. Allow 30 to 45cm between plants at first, but an established plant can reach 1m wide.

It wants moisture, not standing water, so it suits clay that stays damp and drains slowly, not a spot that floods for weeks. See the lovage growing guide.

Quince

Quinces tolerate a range of soils but prefer deep, fertile, moisture-retentive soil, and clay and loam are listed as suitable. Quince is rated H5, hardy to between −15 and −10°C, which is the figure to take seriously in a cold inland garden. No soil preparation is generally needed before planting, and an established tree in moisture-retentive soil should not need watering, though trees on free-draining soil may need extra water.

The bigger Scottish constraint is warmth, not soil. The spring flowers are susceptible to frost damage and sun is needed to ripen the fruit, which is harvested in October or November. Quinces usually crop well in open ground in southern England or milder urban or coastal sites, but further north or in colder or more exposed sites they are best planted against a south- or south-west-facing wall (RHS). The fruit is not edible raw but can be cooked. In a cool Scottish summer the fruit can be left under-ripe, which is why the wall matters more than the soil. Quince is more forgiving of wet ground than damson, and in a very wet garden it can go on a mound. One tree is enough to fruit. A tree on 'Quince A' rootstockThe root system and lower stem a fruit tree is grafted onto, chosen to control the tree's eventual size rather than for its own fruit. Full definition → is semi-dwarfingHow much a fruit tree's rootstock limits or allows its eventual size, from extremely dwarfing (smallest) through to very vigorous (largest). Full definition → and 'Quince C' and 'Quince Eline' are dwarfing, and trees can reach 3 to 4.5m in height and spread. Plant a bare-root tree between November and March. See the quince growing guide.

Comparison

What this doesn't solve

To tell workable clay from waterlogged ground, dig a hole about 30cm deep, fill it with water and see what happens. If it drains within a few hours, you have slow, heavy clay, which is what these five suit. If water is still in the hole a day later, that is standing waterlogging, and none of these five fixes it. Damsons, lovage and quince all want moisture without standing water. The next step is improving clay soil.

Rhubarb also suits heavy, moisture-retentive clay: see the rhubarb growing guide. Brassicas are not ruled out completely, since clubrootA soil-borne fungal disease of the brassica family that swells and distorts roots, and survives in soil for years. Full definition →-resistant varieties exist and cut the risk, so a resistant cabbage or kale on your driest bed is a reasonable compromise.

Common questions

Sources