Nigella Problems: Seedling Losses, Aphids and Rotting Pods

Almost everything goes wrong at the seedling stage

A mature nigella is one of the more trouble-free plants in the garden. The seedlings are not. Because the seed takes up to three weeks to germinate and then produces thread-fine leaves, the plants spend a long time small enough for slugs and snails to clear a whole drift overnight, and because you cannot replace losses by transplanting, a bad night can cost you the entire sowing.

Slugs and snails are the main culprits, and the damage is unmistakable: the seedlings are simply gone, with a few chewed stumps and slime trails left in the morning. Check with a torch after dark for the first fortnight after emergence and remove what you find, and keep the surrounding ground clear of the debris they shelter in. A ring of sharp grit around a small sown patch helps a little; the torch helps a great deal more.

Damping off is the other seedling loss, and it belongs to sowings started under cover or sown far too thickly outdoors. The seedlings collapse at soil level a few days after emerging, as if pinched. It is caused by wet, still, crowded conditions and cured only by prevention: sow thinly, water less, use fresh compost if you are sowing in modules, and give the seedlings all the air you can. Once a patch begins to collapse it will usually continue.

Aphids go straight for the buds

The one insect that troubles a growing nigella is aphid, and it heads for the top two or three inches (5 to 8 cm) of stem just below a developing bud. Colonies build fast in warm still weather, distort the bud so it opens crooked or not at all, and leave honeydew on the foliage below that goes on to grow sooty mold. On a plant grown for its pods this matters more than it looks, because a distorted bud gives a misshapen pod.

Deal with them by hand. Squash the colony between finger and thumb, supporting the stem with your other hand so the hollow stem does not snap, or knock them off with a jet of water. Repeat every two or three days for a fortnight rather than doing it once thoroughly, since a few survivors rebuild a colony inside a week. The stems are brittle, so be more careful than you would be with a woody plant.

Do not spray. Nigella is a good plant for hoverflies and bees, its seed is often taken indoors for the kitchen, and the aphid colonies themselves draw in the ladybirds and hoverfly larvae that clear them within a fortnight. A spray removes the predators along with the aphids and commits you to spraying weekly for the rest of the summer.

1. Check the stem tips whenever you walk past Look at the two or three inches (5 to 8 cm) below each bud. Colonies start there and are trivial to remove in the first few days.
2. Squash or hose them off, supporting the stem Nigella stems are hollow and snap easily. Hold the stem with one hand and run the other up it, or use a gentle jet of water.
3. Repeat every two or three days for two weeks One thorough go never finishes it. The repetition is what actually breaks the colony's rebuilding cycle.
4. Leave ladybird and hoverfly larvae where they are The spiky gray and orange grubs on the stems are ladybird larvae eating the aphids for you. Removing them makes the problem worse.
5. Cut off badly distorted buds A bud that has been fed on heavily opens misshapen and gives a poor pod. Cut it back to the fork below and let the plant branch again.

Aphids do not usually justify any action at all on a plant that is already flowering well. Intervene when the buds are being distorted, not merely because you can see insects on the stems.

Wet ground and wet pods do the real damage

Nigella wants dry feet at both ends of its life. In heavy ground, and particularly over a wet winter after an autumn sowing, the taproot rots and the plant either fails to appear in spring or wilts suddenly and completely once the weather warms. There is no treatment: pull the affected plants, and next time either work grit into the ground before sowing, raise the bed, or move to a spring sowing.

The other wet-weather loss comes at the very end, when the pods are ripening. A run of wet late-summer weather leaves the inflated pods holding water, and gray mold develops inside them, sticking the seed together and turning it dark and lifeless. Plants lying flat after a storm suffer worst of all, which is the practical reason to get support in early. If wet weather sets in while the pods are still green, cut the stems and finish drying them hanging under cover rather than waiting for them to dry standing.

Powdery mildew appears on nigella late in the season, but it usually arrives on a plant that is already finishing. Unless it hits a young successional sowing, it is not worth treating; the plant has done its work. If it does take a young stand, thin or clear the crowded plants, stop overhead watering, and rely on the next sowing rather than trying to save the current one.

Note the first date you saw each problem

The most useful record is the date slugs first took seedlings, the date aphids first appeared on the buds, and the date any mildew showed, together with which part of the garden it happened in. These dates repeat closely from one year to the next in the same garden, and once you have them you can start torch checks or begin looking at stem tips a fortnight before last year's date rather than after the damage.

Note which sowings failed outright and why. Because nigella cannot be replaced by transplanting, a lost sowing means a gap in the flowering season, and knowing that the first sowing is always the one the slugs take is enough reason to sow that patch twice and thin the winner.