Coriander (Cilantro) Care: Watering, Feeding and Delaying the Bolt

Dry roots start the flower stem

Coriander reads drought as the end of its life and responds by flowering. One afternoon of serious wilting in warm weather is enough to set the process going, and once the flower stem has started nothing brings the plant back to leaf. This is the single most common reason a plant bolts three weeks after sowing.

Water deeply and less often rather than sprinkling daily. Let the top half inch (1 cm) of soil dry, then water until it runs from the bottom of the pot or until the ground is wet four inches (10 cm) down. Shallow watering keeps the taproot short and makes the plant more, not less, prone to drying out.

1. Water in the morning A plant that goes into the hottest part of the day with wet roots does not wilt. Evening watering leaves the foliage damp overnight and encourages leaf spot.
2. Mulch as soon as the seedlings are thinned An inch (2 to 3 cm) of straw, leaf mold or fine bark keeps the surface cool and roughly halves how often you need to water in summer. Keep it clear of the stems.
3. Check pots twice a day in a heatwave A pot under eight inches (20 cm) across can go from moist to bone dry between breakfast and mid afternoon. If you cannot check it, move it into shade rather than risk the wilt.

Wilting at midday that recovers by evening is normal in strong sun. Wilting that is still there at dusk means the roots are genuinely dry, and that is the wilt that triggers bolting.

Heat, not day length, is what pushes it into flower

Long days do push coriander towards flowering, but temperature is the stronger lever in a garden. Sustained days above 77F (25C), and especially nights that stay above 68F (20C), will bring a plant into flower within two or three weeks whatever else you do. Below about 68F (20C) the same plant can stand in leaf for two months.

You cannot cool a garden, but you can choose where the plant sits. Coriander is one of the few crops that genuinely prefers part shade in summer: four or five hours of morning sun with shade from midday onwards gives noticeably more leaf and a later bolt than an open, sunny bed.

Growing coriander in the shade of taller crops such as beans or sweetcorn works well and costs no extra space. Sow the row on the shaded side once the tall crop is up.

Feed lightly, and stop before the plant is large

Coriander is a short lived plant and does not need much. A soil with plenty of organic matter and a modest amount of nitrogen carries a leaf crop from sowing to cutting without any feeding at all. Overfeeding gives you soft, floppy growth that aphids find first and that collapses in the first wet spell.

If the leaves are pale and the plant is standing still, give a dilute liquid feed once every two or three weeks while you are cutting. That is the whole feeding program. Plants grown on for seed need even less: too much nitrogen late on delays ripening and gives you a lot of leaf and few umbels.

Containers: depth beats width

Because the plant roots straight down, a deep, narrow pot suits it better than a shallow, wide one. Eight inches (20 cm) of soil depth is the working minimum and ten to twelve inches (25 to 30 cm) is noticeably better. In a shallow tray the taproot hits the bottom, turns sideways and the plant flowers early regardless of the weather.

Use an ordinary all-purpose potting mix rather than a very free draining mix. Coriander wants moisture held around the roots, and a gritty mix dries out between waterings in exactly the way that triggers bolting.

1. Sow into the pot, not into it later Fill the final container, water it, and sow into the damp surface. Coriander should never be potted up.
2. Space them properly even in a pot Three or four plants in an eight inch (20 cm) pot is the limit. A dense clump of a dozen seedlings runs out of water daily and bolts as one.
3. Use a light colored pot in summer A dark plastic pot in afternoon sun can push the rootball above 95F (35C), which alone will start the flower stem. Pale pots, or a pot standing inside another, stay cooler.

Standing pots on a saucer of gravel with a little water in it steadies the moisture on hot days. Do not stand the pot directly in water, though, or the lower roots suffocate.

Reading the plant

Coriander tells you what is wrong through the shape of its new growth as much as its color. The first leaves are broad and scalloped like flat leaf parsley; the leaves that come after a plant decides to flower are fine and thread like. Learning to spot that change a week early is what lets you cut a good crop before the plant is finished.

Once the feathery leaves appear the plant is committed. Pinching out the flower stem buys a few days at most and the leaf that follows tastes soapy and thin. Sow the next row instead.