When to sow phacelia

Phacelia is a fast annual from California and the south-west of North America. Sow it outdoors from April, once the soil is workable, until early September. In mild parts of the UK you can start in March.

What you sow it for decides the date. For flowers, sow from April to June and it will bloom from early summer. As a green manure, it fits any gap of six weeks or more: after early potatoes, peas or salads in summer, or on a bed that would otherwise stand empty over winter.

Sowings from late August or early September grow on until frost. Harder frosts kill the plants and the dead stems lie on the soil as a winter cover; in mild winters, late sowings sometimes survive.

Rake the bed to a fine surface. Scatter the seed thinly, about 1–1.5 g per m², then rake it in to 0.4–0.8 in and firm the soil. The seeds need darkness to germinate, so don't leave them on the surface. You can also sow in rows about 6 in apart.

A 120 × 80 cm raised bed is almost exactly one square metre, so about 1–1.5 g of seed covers it. Weigh it out once to get a feel for how little that is. Sown too thickly, the plants stay spindly.

Keep the soil moist until the seedlings are up, which takes about one to two weeks. After that phacelia needs little: no feeding, and water only in a long drought.

Phacelia starts flowering about six to eight weeks after sowing and, depending on when you sowed, blooms from June into October. The curled flower heads open bit by bit from the base, so each planting flowers for weeks.

The flowers produce plenty of sugary nectar and pollen. Honeybees, bumblebees and hoverflies visit them in large numbers, which is why beekeepers and farmers sow it widely. Hoverfly larvae eat aphids, a bonus in a vegetable garden.

If you grow it for bees, let it flower as long as it likes, but cut it down before the seeds ripen if you don't want it all over the garden next year.

As a green manure, phacelia covers bare soil fast, smothers weeds, and holds on to nutrients that rain would otherwise wash out between crops. It also helps keep the soil in good condition. Unlike clover or vetch it is not a legume, so it does not add nitrogen from the air.

Cut or mow it before it sets seed, usually at or soon after the start of flowering. Chop the stems and either leave them on the surface as a mulch or dig them lightly into the top layer of soil.

Wait three to four weeks before sowing or planting the next crop in that spot: fresh, rotting green matter can hold back seedlings.

Autumn sowings can simply be left. The frost-killed plants protect the soil over winter, and in spring you rake off what's left or work it in shallowly.

Phacelia's great advantage in a vegetable garden: it belongs to the borage family (Boraginaceae; older books put it in the waterleaf family, Hydrophyllaceae) and is related to none of the usual vegetables. It therefore carries over no brassica, legume or onion-family diseases, and you can slot it into any rotation. That is exactly why it is so popular in Germany as a gap filler.

Real problems are rare. The main risk is self-seeding if you let it ripen. The plant contains substances that can irritate sensitive skin, so wear gloves when you cut a large patch.

Phacelia is often sown along or between vegetable beds to draw in bees and hoverflies. That is a sensible use of the flowers; a direct effect on the yield of neighbouring crops has not been shown in garden trials we could find.