🌱✗ Tested and false“Kohlrabi is a root vegetable, like a turnip or swede”
RHS describes it precisely: kohlrabi 'forms a swollen stem base, often called a bulb', not a root. It develops above the soil surface, unlike turnip or swede, which swell underground.
Our read The confusion is understandable, since it eats and tastes like a turnip and sits low in the bed the same way, but the part you're growing is stem tissue, not root tissue.
What to do: Nothing changes in practice, since the growing advice is similar to a root crop either way, but it's worth knowing when a source talks about root disturbance or transplanting: kohlrabi's swollen part isn't underground, so covering or mounding soil over it doesn't help it the way earthing up helps an actual root or tuber.
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- RHSKohlrabi forms a swollen stem base, often called a bulb, which can be green, white or purple on the outside
“Purple and green kohlrabi are the same vegetable, just a different colour”
Two independent sources agree on a real growing difference: RHS and Gardener's World both say purple varieties are hardier and slower-growing than green, which matures faster.
Our read Colour looks purely cosmetic on the shelf, but it's tracking a genuine difference in how the plant handles a late or unsettled sowing, which matters more in a Scottish shoulder season than it would somewhere with a longer settled summer.
What to do: Choose green varieties, such as RHS's 'Korist', for a fast spring crop, and save purple varieties, such as 'Kolibri', for a late-summer sowing where hardiness through less predictable weather matters more than speed.
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- RHSPurple varieties are hardier and slower to mature, ideal for sowing in late summer
- Gardener's WorldGreen varieties mature faster (March-June), while purple types are hardier and better for summer sowing
“Leave kohlrabi in the ground longer to grow a bigger, better bulb”
All three institutional sources agree: RHS, Garden Organic and Gardener's World each independently say to harvest at golf-ball to tennis-ball size, warning that anything larger turns woody and tough.
Our read It's the same intuition that applies to a squash or a marrow, where bigger genuinely can mean more of the vegetable to use, but kohlrabi runs the other way. This is the rare case where three sources gave us the exact same warning without prompting, which is about as settled as a claim gets.
What to do: Check plants every few days once bulbs start sizing up, since Garden Organic's advice is blunt: 'the plants mature quickly'. Pull at golf-ball to tennis-ball size rather than waiting for it to look more substantial.
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- RHSHarvest kohlrabi when young, between golf-ball and tennis-ball size, to enjoy them at their sweetest
- Garden OrganicDig up bulbs when still small (golf ball to tennis ball size); check regularly as the plants mature quickly
- Gardener's WorldHarvest when stems have reached golf to tennis ball size, any larger and they become tough and woody
“Kohlrabi only bolts if the weather turns cold”
Bolting has three separate triggers, not one: heat, drought, or cold. The risk is highest in hot, dry conditions, not just a cold snap.
Our read Cold gets blamed because it's the trigger most gardeners have heard of for brassicas generally, but for kohlrabi specifically, a dry hot spell is at least as likely a cause.
What to do: Keep the soil consistently moist through any dry spell, not just in cold weather, since drought stress is named as a bolting trigger in its own right, alongside making the bulb woody.
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- RHSBolting is caused by heat, drought, or cold; risk is highest in hot, dry conditions
“Kohlrabi has to go in open ground — it won't work in a pot”
RHS gives container growing as a named alternative, specifically recommended for gardeners short on ground space: a pot at least 30cm wide and deep, in sun or semi-shade, filled with peat-free multi-purpose compost, thinned to 15cm spacing.
What to do: If you don't have a spare bed, use a 30cm-plus pot rather than skipping the crop. It's suggested specifically as a way to grow a quick crop of mini veg.
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- RHSKohlrabi is best sown in the ground, but if you're short on ground space, you could sow in a large container, especially to harvest as a quick crop of mini veg. Choose a pot at least 30cm (1ft) wide and deep.
“Dig sand or grit into heavy clay to break it up”
The quantities involved make this hopeless at garden scale. Meaningfully diluting a soil that is 40 per cent clay takes around 250kg of grit or gravel per square metre. A barrowload does nothing, and adding modest amounts can destabilise the structure and leave the ground harder to work than before you started.
What to do: Add bulky organic matter instead: manure or composted bark, dug in generously. Raise beds to lift roots above the wet, and consider not digging at all. Those three do what the sand was supposed to do, and they compound each year rather than needing repeating.
In Scotland: Heavy clay runs right through the Central Belt, so this is one of the most expensive mistakes a Scottish grower can make. Two tonnes of grit for a small bed is not a purchase anyone makes twice.
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- RHSRoughly 250kg per sq m of grit needed; adding material can make clay less stable
“You have to dig the ground over each year to keep the soil fertile”
Digging works against fertility rather than for it. Turning the ground releases soil nitrogen and damages the soil's biology. The structure that fungi, worms and roots build up over a season is undone every time the spade goes through it.
What to do: Leave the ground alone and feed it from the top. Spread compost or well-rotted manure on the surface and let the soil life pull it down. Dig only where you actually need to, such as lifting root crops or clearing perennial weeds.
In Scotland: There is a practical bonus here: undug ground carries weight better and warms marginally earlier in spring than freshly turned wet soil, which matters when the season is already short.
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- RHSDigging releases soil nitrogen and damages soil biology
- RHSRecommends no-dig among the practical options for heavy ground
“Peat-free compost doesn't grow plants as well as the peat stuff”
This was a fair complaint a decade ago and it has not aged well. Peat-free quality has greatly improved in recent years, and dedicated research has found some carnivorous plants grow better in peat-free mixes than in peat. What is true is that peat-free behaves differently: the watering and feeding requirements are not the same, because the ingredients have different physical and chemical properties from peat.
Our read Most bad experiences with peat-free are a watering problem rather than a compost problem. It dries differently, so the habits you built on peat will let you down for a season until you adjust.
What to do: Read the bag rather than assuming, since feeding and watering advice genuinely varies between mixes. Buy fresh: old stock breaks down and performs worse regardless of what it is made of. If one brand disappoints, try another rather than writing off the category.
In Scotland: This matters more here than almost anywhere in Britain. Scotland holds a great deal of the UK's peat, those peatlands are major carbon stores, and peat-based compost comes out of them. The performance excuse for buying peat no longer holds up.
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- RHSQuality has greatly improved; own research shows some plants do better peat-free; watering and feeding requirements differ
“Plants do better in pots than in the ground”
A container limits how far roots can spread and cuts the plant off from the reservoir of moisture and nutrients that open ground provides, increasing the need for both watering and feeding. You are taking on work the soil would have done for free.
Our read Pots are a solution to a problem, not an upgrade: no ground, bad drainage, a crop that needs containing, or somewhere warm against a wall. Chosen for those reasons they are excellent. Chosen by default they make the job harder.
What to do: Use the ground where you have it. Where you do use pots, go bigger than feels necessary, expect to water far more often, and feed through the season because there is no soil reserve to draw on.
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- RHSPot planting causes limited root spread and increases the need for watering and feeding
“Strong-smelling plants confuse pests and hide your crop”
This is the theory underneath half of every companion planting chart, and it is the wrong mechanism. Research at Wellesbourne, Britain's long-standing centre for brassica and carrot pest work, found that insects hunting a host land on green and avoid bare brown soil. Colour, size and shape decide where they settle. Artificial green plants, and even green paper releasing nothing at all, disrupt them as well as living companions do, and aromatic plants perform no better than bland ones.
What to do: Stop choosing companions by scent. If ground cover is what disrupts the pest, the useful question is whether something low and green grows between your rows, not whether it smells of anything.
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- Warwick Crop CentreArtificial green plants and green paper are as effective as living companions; aromatic plants no better than less pungent species
“Crushed eggshells, bark mulch or wool pellets stop slugs”
Testing the most popular home slug barriers found no reduction in damage from crushed eggshells, bark mulch or wool pellets. This is Britain's most repeated slug advice and it does not work.
What to do: Skip the barrier. Nematodes, evening hand-collection, and raising plants to a decent size in modules before planting out all do more.
In Scotland: This one costs us more than most. A wet, mild climate is close to ideal for slugs, so a barrier that does nothing is a bigger loss here than in a dry southern garden where the pressure is lighter anyway.
2 sources
- RHSNo reduction in slug damage from these barriers
- Garden OrganicRecommends nematodes and hand-picking over physical barriers, which it does not endorse
“Watering in sunshine scorches the leaves”
Water droplets do not focus sunlight enough to burn foliage. Scorch on new plants comes from insufficient hardening off far more often.
What to do: Water whenever the plant needs it. Watering at ground level rather than over the leaves is still worth doing, but for disease and evaporation reasons, not for scorch.
In Scotland: Rarely the limiting factor here anyway, though it matters under cover, where watering habits drive humidity and therefore disease.
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- RHSScorching from lack of hardening off is far more common
“Coffee grounds acidify your soil and feed your plants”
Grounds do contain nitrogen, phosphorus, potassium and magnesium, so the feeding half is not baseless. The acidifying half is not: their effect on pH is very minor and temporary. Spread thick they cap the soil and block water and air, and residual caffeine can suppress root and shoot growth.
What to do: Put them in the compost bin rather than straight on the bed. If you actually need to change pH, that is a job for lime or sulphur guided by a soil test, not for coffee.
In Scotland: Most Scottish soils are naturally acidic already, so the acidifying claim solves a problem you probably do not have and may worsen one you do.
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- RHSEffect on pH is very minor and temporary; caffeine can suppress growth
- WSU Extension — Linda Chalker-Scott, WSU Extension Urban HorticulturistA commonly held assumption states that coffee grounds are acidic, but this does not hold true experimentally... the pH of decomposing coffee grounds is not stable and one shouldn't assume that it will always, or ever, be acidic.
“Copper tape round your pots keeps slugs off”
Two authorities we trust reach opposite conclusions on Britain's most popular slug barrier. RHS ran its own research and found no reduction in slug damage from copper tape or sharp grit in garden conditions, and says plainly that many barrier methods have no scientific evidence behind them. Garden Organic calls copper tape the most effective barrier for container growing, though expensive, and reckons copper banding on pots tends to work well.
Our read The disagreement narrows once you look at where each was testing. RHS tested garden conditions; Garden Organic's endorsement is specifically about containers, where a continuous unbroken ring is actually achievable. If you are going to try it, pots are the only place it has a supporter.
What to do: Don't ring an open bed with it and expect much. On containers it is at least defensible, but check the tape stays unbroken and clean, and don't let a leaf bridge it. Nematodes remain the one control both authorities endorse.
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- RHSOwn research found no reduction in damage from copper tape or sharp grit
- Garden OrganicThe most effective barrier for container growing, though expensive
“Beer traps will deal with your slugs”
Beer traps are listed among trapping options but with no claim that they work, and we could find no trial data from either authority establishing how much difference they actually make.
Our read They certainly catch slugs, which is why gardeners believe in them, but a trap full of slugs is not the same as a bed with fewer. Beer also attracts slugs in, so a trap in the middle of a seedling bed may be drawing more traffic than it removes. If you use them, put them at the edges rather than among the plants.
What to do: Treat them as a way of seeing what you have rather than a control. Sink the rim above soil level so ground beetles, which eat slug eggs, don't drown in them.
In Scotland: Our wet weather fills and dilutes open traps quickly, so they need emptying more often here than the advice usually assumes.
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- RHSListed as a trapping option, with no effectiveness claim made
“The leaves you trim off kohlrabi are just waste, only the bulb is worth eating”
Two independent sources say otherwise. Garden Organic confirms the leaves are harvestable alongside the bulb, and Gardener's World specifically says they can be steamed like spinach, which RHS's page doesn't mention at all.
Our read This is exactly the kind of fact a single source can miss and a wider read catches: RHS is silent on the leaves, and if that were the only page consulted, this would have gone down as an open question rather than a two-source, independently corroborated yes.
What to do: Don't compost the leaf trim from a kohlrabi you're harvesting. Steam or wilt it like spinach or spring greens, the same day if possible, since it won't keep as well as the bulb.
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“Slug nematodes actually work”
This is the one slug control both our authorities endorse. The nematode Phasmarhabditis hermaphrodita is watered into moist soil between 5 and 20°C, spring to early autumn, with effectiveness lasting at least six weeks per application.
Our read Worth knowing before you buy: RHS points out it affects all slug species, including the ones doing useful work breaking down dead matter. That sits awkwardly beside the same organisation's advice to tolerate slugs where you can, and it is a reason to treat seedbeds rather than the whole plot.
What to do: Apply to moist soil once it has warmed past 5°C, and follow the watering instructions rather than treating it as a sprinkle. Garden Organic found control varies by crop, good on some and only intermediate on lettuce, so expect help rather than elimination. It is expensive, which is another argument for targeting it.
In Scotland: The soil temperature requirement is the catch here. In much of Scotland the ground does not hold above 5°C reliably until well into spring, which is later than the packet tends to imply and often after the first seedlings have gone in.
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- RHSMoist soil at 5-20°C, spring to early autumn; affects all slug species
- Garden OrganicEffective at least six weeks per application; control varies by crop; expensive
“Mulching is worth the effort”
This one earns its reputation. A layer of organic mulch conserves moisture, keeps the surface from cracking, suppresses weed germination by keeping light off the soil, and feeds the ground as it breaks down. It is worth doing directly for heavy soils, where it does more good than the grit people usually reach for.
What to do: Mulch onto ground that is already moist, not baked dry, or you seal the dryness in. Keep it clear of stems, and use compost, well-rotted manure or composted bark rather than anything that has not broken down.
In Scotland: Our rainfall means moisture retention matters less here than it does south, so the weed suppression and the slow feeding are the real prizes. On heavy ground a surface mulch also spares you digging wet soil, which does more harm than the weeds would.
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- RHSOrganic mulches around plants reduce cracking and conserve moisture
- RHSMulching saves water, suppresses weeds and improves soil structure
“Growing flowers among your plants brings in insects that eat the pests”
Hoverflies and lacewings eat aphids as larvae but live on nectar and pollen as adults, so a plot with nothing in flower starves the insects doing your pest control. UC IPM is clear that this works and equally clear about its limits: the benefit falls away with distance from the flowers, and it will not always show up as fewer pests.
Our read This is the one piece of companion-planting folklore that survives, and it survives because it works by feeding predators rather than by confusing pests with smells. Note what it is not: a guarantee, or a replacement for mesh where a pest is serious.
What to do: Put the flowers close among the crop rather than in a border at the far end of the plot, since the effect is local. Aim for something in flower across the whole season rather than one big flush.
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- UC IPMNectar and pollen sustain adult natural enemies; benefit declines with distance and may not always increase biological control
