Kohlrabi Care: Watering, Feeding and Never Earthing Up

The bulb sits on the surface, so never earth it up

The part you eat is not a root. It is the stem, swollen just above the soil, and it grows sitting on top of the ground with the leaf stalks radiating off it. Everything about how you look after kohlrabi follows from that.

The mistake almost everyone makes at least once is to draw soil up around it the way you would with a leek or a potato. Buried, the swollen stem does not blanch or sweeten. It rots at the contact point, or it puts out roots from the buried section and stops swelling. If soil or heavy mulch has drifted up against a bulb, pull it back with your fingers.

The same logic applies to weeding. Hoe shallowly and away from the plant. A hoe run in close throws soil onto the bulb and nicks the surface roots that sit in the top two inches (5 cm).

If a bulb has grown lopsided with one flat side, it was either sitting on a lump of soil or leaning after wind. Neither is fixable once it has started, so level the bed before sowing.

Even water is what makes it crisp

Kohlrabi texture is decided almost entirely by whether the water supply was steady. A plant that dries out and then gets a soaking splits, and a plant that is dry for a long stretch turns fibrous and hot tasting. Neither can be reversed.

It is a shallow rooted crop, so it has no reserve to draw on. In the fortnight when the bulb is actually swelling, from about an inch (2.5 cm) across to full size, it needs the most water it will ever need. That is when a dry spell does the damage.

Water in the morning at the base of the plant. Water sitting in the crown of leaves overnight in cool weather is how leaf rot starts on a crop that has no height to it.

Feed early and lightly, then stop

Kohlrabi is in the ground for a short time and does not want a heavy feeding program. It wants enough nitrogen to build its leaves in the first month, and then very little. Feeding late pushes soft leafy growth at the expense of the bulb, and soft growth is what aphids find.

1. Work compost in before sowing A bucket of well rotted compost per square yard, forked into the top six inches (15 cm), covers most of what the crop needs. Fresh manure gives you lush leaves and a small bulb.
2. Feed once at the second thinning When you take the plants down to their final spacing, scatter a balanced feed along the row, a few inches (5 to 8 cm) away from the stems, and hoe it in shallowly. This is the feed that builds the leaf area that fills the bulb.
3. Stop feeding once the stem starts to swell Once you can see the stem thickening at the base, stop. Nitrogen after that point gives you a big leafy plant with a small, soft bulb that will not keep.
4. Correct pale leaves, ignore large ones Uniformly yellow-green young leaves mean it is short of nitrogen and a liquid feed is worth it. Large dark leaves with no bulb mean the opposite, and the answer is to stop feeding and water evenly.

Kohlrabi wants a soil around pH 6.0 to 6.8, like other brassicas. Acid soil stunts it and makes club root far more likely, so lime a season ahead if your soil is below that.

What to do with the leaves

The leaves are the engine. Each one feeds the bulb, so do not strip them off in the belief that it helps the bulb swell, the way you might thin fruit on a tree. It does the opposite.

There are only two reasons to remove a leaf. The lowest leaves go yellow and floppy as the plant matures and are worth taking off, because they lie on the soil and hold damp. And any leaf with a caterpillar colony or heavy aphid coverage comes off entirely rather than being treated.

1. Take yellowing bottom leaves off cleanly Snap or cut them right back at the stem rather than leaving a stub. A stub rots and marks the bulb surface underneath it.
2. Keep at least six healthy leaves Below about six working leaves the bulb stops growing. If pests have taken you down to that, deal with the pest before you remove anything else.
3. Clear the fallen leaves off the bed Leaves left lying next to the bulb are where slugs shelter during the day and where rot spreads from in wet weather.
4. Use them Kohlrabi leaves cook like kale and are best from young plants. Taking the odd leaf for the kitchen from a plant with plenty costs you nothing.

Wind, heat and what is worth recording

A kohlrabi plant is top heavy on a narrow base. Wind rocks it, the stem loosens in its socket, and the bulb grows crooked with a torn skin on one side. In an exposed spot, run a low windbreak of mesh along the windward side of the bed rather than staking individual plants.

Heat is the other stress. Above about 86F (30C) the plant slows, and a bulb that sits through a hot spell turns fibrous even with enough water. Shade cloth over a late summer sowing during the hottest fortnight buys you a better texture.

1. Note the sowing date and the date the bulb reached an inch (2.5 cm) The gap between those two tells you when to start checking for harvest next year, and it is far more useful than a generic days-to-maturity number.
2. Note where in the bed the brassicas went Kohlrabi should not follow another brassica on the same ground for three years, and after two seasons you will not remember. A sketch of the bed with the positions marked is enough.
3. Note the first date you saw flea beetle damage It arrives at roughly the same point each year. Knowing that date means you cover the next sowing before it happens rather than after.
4. Note which plants bolted and when they were sown Bolting traces back to the sowing date and the cold spell that followed. One season of records tells you which end of your sowing window is safe.

Kohlrabi occupies ground for only two to three months, so it slots in before or after a longer crop. Write down what it followed as well as what followed it, because the rotation is what keeps club root out.