Growing herbs

Five of the nine are woody shrubs and subshrubs that will outlive the bed they are planted in. They share one habit that decides how you treat them: they will not reshoot from old bare wood. Left unpruned, each year’s growth starts a little higher up, the base goes bare and brittle, the middle opens out, and cutting back into that wood does not bring the plant back. Prune lightly every year, from the first year, always into growth that still carries leaves.

They also all want the same things, which makes them the easiest group in the garden to manage: one dry, sunny, sharply drained corner, watered while they establish and then largely left alone. Put them together and there is no compromise to make.

Four are sown fresh rather than kept: basil, cilantro and dill are annuals, and parsley is a biennial that makes leaves in its first year and runs to seed in its second. None of them can be rescued once it flowers, so the technique is not to keep a plant going but to have the next sowing already coming.

Sowing again every two or three weeks is worth more than anything else you can do for cilantro and dill in particular. Both bolt as soon as the weather turns, and a single large sowing gives a glut and then nothing. Three of the four also resent transplanting — dill and cilantro have taproots that bolt when disturbed — which is why a pot from the supermarket so rarely settles into a bed.

Mint, which we have no guide for yet, is the one nobody should plant in open ground. It spreads by underground runners and does not stop. A mint put into a bed will be in the rest of that bed within two seasons and is genuinely difficult to remove, because every fragment of root left behind regrows. Grow it in a container, standing clear of the soil so runners cannot reach over the rim. This is the only herb where the advice is a warning rather than a method.

Marigold sits in the herb group in our data and is not a herb at all — nothing on the plant goes in a kitchen. It earns its place beside them for a different reason: grown as a dense stand and dug in, it genuinely suppresses root-knot nematodes, which is one of the few companion planting claims that survives testing. Its guide covers what it does and does not do.

• Full sun. Six or more hours. Shade gives longer, softer, sparser growth with noticeably less flavour, because the aromatic oils build in sunlight. Cilantro is the single exception and prefers afternoon shade once the weather warms.
• Sharp drainage above everything else. More woody herbs are lost to winter wet than to cold. On heavy ground, plant on a mound, in a raised bed or in a pot, and push the mix towards the gritty end. Do not dig a gravel pocket into clay — it fills with water and holds it against the roots.
• No feeding, for the perennials. Rich soil and nitrogen give a big soft plant with weak flavour. The soft annuals are the exception and take a light feed.
• No water-holding mulch against the stems. A generous organic mulch in autumn keeps the crown damp through exactly the months it needs to be dry. Grit or gravel is fine.
• Pruning into leafy growth only. Never into the bare brown wood on any of the five perennials.

No publication behind these pages gives a soil pH for an individual herb. UGA puts most herbs at about 6 to 7.5 and Minnesota at 6.0 to 7.5, which agree, and lavender and rosemary are singled out as taking the alkaline end. Each guide says so in its own quick facts table rather than printing a figure for the crop.

Herbs are the most-recommended companion plants in a vegetable garden, and the reasons given are a mixture of one well-supported mechanism and a great deal of folklore. The mechanism that holds up is that small open flowers feed the beneficial insects whose larvae eat aphids and caterpillars — which means letting some of the dill, cilantro, oregano and thyme actually flower rather than clipping it all for the kitchen.

The other real effect is the one this page opened with, and it runs the other way: a woody herb next to a thirsty vegetable will be killed by the watering that vegetable needs. The companion planting chart shows both, crop by crop, and labels how well each pairing is supported.