How to Grow Phacelia in Australia

Phacelia (Phacelia tanacetifolia), also called tansy phacelia, is a fast annual in the borage family, Boraginaceae. It is grown to feed bees and beneficial insects, and as a cover crop that is cut and dug in to feed the soil. It fills a resting vegie bed in autumn or spring, and the usual mistake is leaving the seed uncovered.

Three mistakes that matter

1. Leaving the seed uncovered. Phacelia seed germinates in darkness, so seed scattered and not raked in often fails to come up. Cover it with 5 to 6 mm of soil and firm it down, as Sowing Seeds describes.
2. Sowing into warm soil. The seed germinates poorly in soil warmer than the range When to plant gives. Midsummer and tropical sowings often come up patchy or not at all. Sow in autumn or spring instead.
3. Cutting it down too late. Once plants set seed, the soil benefit falls and seedlings pop up all over the bed next season. Cut it at the time Flowering and Cutting Down gives.

When to plant

Phacelia is a cool-season crop. The seed germinates in soil from about 4°C to 20°C, so sow in autumn to cover a bed through winter, or in spring for flowers. Established plants take light frosts, but frosts below about minus 7°C kill them.

Autumn sowings grow slowly through winter and flower from late winter into spring. Spring sowings flower sooner, usually 6 to 13 weeks after sowing.

Site, soil and setup

Phacelia asks for little: sun, drainage and a weed-free seedbed. Rich soil is not needed, because a cover crop is there to gather spare nutrients rather than to be fed.

Container growing

Choose a pot at least 30 cm across, about 13.5 L, and fill it with quality potting mix. The store sells one form of phacelia, not a dwarf selection, which grows 80 cm to 1 m tall. A pot also keeps the seed from spreading through the garden.

• Sowing: sow a pinch of seed 5 mm deep and thin to three or four plants.
• Water: keep the mix moist, never soggy, and check it often in warm weather.
• Feed: fish and seaweed blend at half strength, about 12 mL per 9 L, fortnightly only if leaves pale.
• Position: full sun or light shade, where bees can find the flowers.

Sowing Seeds

Sow phacelia direct where it is to grow. How thickly and where depends on what the crop is for.

Sowing a cover crop

• Scatter the seed evenly over the prepared bed, as scatter sowing explains.
• Rake it in to 5 to 6 mm deep and firm the soil with the back of the rake.
• Water in gently and keep the surface moist, never wet, until seedlings appear.
• Cover the bed with shade cloth to stop birds taking seed, and lift it once seedlings show.
• Expect seedlings in 7 to 28 days; cool soil is slower, so wait before resowing.

Sowing for flowers

• Sow two or three seeds per spot, 5 mm deep and 15 to 30 cm apart, then thin to the strongest seedling.
• For a border, seed can be raised in cells and planted out while small.
• Harden off cell-raised seedlings (gradually accustom them to sun and wind), as hardening off seedlings describes.

Care by growth stage

Moisture decides this crop. Plants tolerate dry spells once established, but steady water keeps them growing and flowering.

Extra feed is rarely needed, since phacelia grows in nutrient-poor soil. If growth stalls in very poor soil, a light dose of all-purpose controlled release granular (NPK around 14-5-10) scratched in can help.

Companion planting

Phacelia earns its place beside vegetables by feeding the insects that hunt their pests. Hoverflies, lacewings and parasitic wasps all visit it.

Grows well with

• Broccoli and cabbage, where hoverfly larvae from the flowers hunt aphids.
• Alyssum, a low insectary edging that sits beneath the taller phacelia.

Keep apart

• Lettuce and potatoes: phacelia can host the sclerotinia fungus that attacks both, so never grow them beside it or after it in the same bed.
• Carrots and other slow, low seedlings, which a vigorous stand up to 1 m shades and crowds out.

Flowering and Cutting Down

For the soil, cut phacelia as the first flowers open. For bees, let it flower, then cut it as the lowest seed capsules on the first spikes start to brown. Seed ripens low on each spike while the tips still flower.

• Wear gloves and long sleeves, because the stiff hairs on older plants can irritate skin.
• Slash or mow the stems into short pieces, which break down faster.
• Dig the cut tops lightly into the soil, or leave them on the surface as mulch.
• Wait 4 to 6 weeks before sowing seed where tops were dug in.
• To plant sooner, leave the tops as mulch and plant seedlings through them.
• Do not let a stand drop seed: phacelia has naturalised in paddocks and pastures in several states.

To save seed, cut a few heads as the lowest capsules turn brown and dry them in a paper bag. Store the seed somewhere cool and dry.

Cut flowering stems for a vase, where their strong stems give a long vase life.

Common problems

Patchy, slow emergence. Phacelia seed often comes up slowly and unevenly, over 14 to 28 days in cool soil, so wait before resowing. Depth and warm soil are covered in Three mistakes that matter. Read more

Tall, floppy plants with few flowers. Too much shade makes plants stretch and flower poorly, so sow in full sun or light shade.

Plants stall or die in wet beds. Waterlogged soil is the usual cause, since phacelia needs free drainage. Read more

Pests and diseases

Phacelia has few problems, and most arrive at the seedling stage or from wet soil.

Aphids (Aphididae). These small sap-sucking insects cluster on shoot tips, and hoverfly larvae from the flowers can clean them up. Hose them off, and never spray pyrethrum onto open flowers, because it harms bees. Read more

Snails and slugs (Gastropoda). They chew tender seedlings at night, and the grey field slug is common in the higher rainfall south. Collect them by torchlight after rain. Where they are thick, scatter an iron-based bait thinly at sowing, never in heaps. Keep dogs away, since even iron baits can poison a dog. Read more

Sclerotinia rot (Sclerotinia minor). Phacelia can host this soil fungus, which causes a water-soaked rot with white, fluffy growth and small black resting bodies. Pull out and bin affected plants; Keep apart names the crops to keep out of that bed.

Damping off (Pythium species and Rhizoctonia solani). Seedlings rot at soil level and collapse, most often in cold, wet soil. Water less and sow thinner. Read more

Prevention first

• Sow into cool soil in autumn or spring, so seedlings grow fast past the damping off stage.
• Scatter seed evenly rather than thickly, so air moves between seedlings.
• Check new seedlings at dusk after rain for snails and slugs.
• Leave hoverflies and lacewings to work, and keep sprays off flowering plants.

Tips

1. Let a small patch sown in autumn flower through late winter, when it gives bees an early source of pollen.
2. Scatter a pinch of phacelia through a mixed cover crop of brassicas or grains for flowers among the leaves.
3. In vineyards, phacelia sown in every tenth row feeds the beneficial insects that help keep light brown apple moth in check. Try a strip beside a backyard grapevine.