Phacelia: sow, grow

Description and Botany

The phacelia, in Latin Phacelia, is one of those plants known as “green manures”. The genus Phacelia comprises around 150 species of annual, biennial, or perennial plants, but in our gardens, we only cultivate the annual phacelia or Tansy-leaved Phacelia (Phacelia tanacetifolia).

This annual herbaceous plant, belonging to the botanical family Boraginaceae and native to North America, develops a well-developed, dense, and deep root system, which is very useful for aerating and loosening the soil, as well as fixing and returning nitrogen to the soil, a nutrient essential for plants. With very rapid growth, it quickly forms a dense clump, sometimes reaching up to 1 m in height: phacelia rapidly colonises large areas to prevent weeds from growing.

It has a very sculptural upright habit with its rigid, slightly reddish stems covered in hairs. The elegant, deciduous foliage is soft and finely divided, reminiscent of that of tansy or certain ferns. It consists of large, alternate green leaves, about twenty centimetres long, oblong, lobed, and well-spaced along the stems.

Field of phacelia, inflorescence and foliage

On this finely divided vegetation, flowering occurs from May to August. Grouped in cymes, the inflorescences in the shape of a crozier gradually unfold as flowering progresses. They reveal small, fragrant flowers about 1 cm in size, formed of 5 petals in shades of blue mauve to lavender blue. These small inflorescences, encased in a narrow calyx with hairy sepals, release a bouquet of long stamens.

This fragrant, melliferous, and nectariferous flowering lasts about 6 weeks. It attracts pollinators throughout the summer, particularly bees and butterflies, as well as other beneficial insects like hoverflies, bombyliids, and ladybirds, which are excellent predators of aphids and scale insects.

The fruits are small dehiscent capsules containing 4 seeds: the plant can become invasive through spontaneous sowing.

In the garden, phacelia is a sovereign plant: it is essential in the context of an ecological garden or permaculture. This green manure is appreciated for its ability to aerate and fertilise soils, limit weeds, its melliferous qualities which are very useful in the vegetable garden and orchard, and also for its beautiful rustic appearance. It primarily serves as an intermediate crop, which is why it is particularly useful in a crop rotation vegetable garden to allow the soil to rest and regenerate from year to year.

Main species

Sowing of phacelia

Where to Sow It?

Phacelia is a frost-sensitive plant, so it is grown as an annual in the garden. Easy to cultivate, it thrives in all soils, even the most ungrateful and driest. In terms of exposure, it prefers full sun.

It is sown as a green manure in crop rotation in the vegetable garden on an uncultivated plot or between rows of vegetables. It can also be grown in areas where you wish to limit grass growth or eliminate “weeds.”

Highly melliferous, Phacelia is also very useful in orchards. It is an aesthetically pleasing and fragrant plant that can be sown in a naturalistic garden mixed with other annual flowers, in wildflower beds, or in natural meadows, where it will attract pollinators.

Phacelia has its place in the vegetable garden, here as a second crop (Photo: N. Doty), but also in naturalistic gardens or meadows (Photo: A. Slater)

Why Sow It?

Phacelia is beneficial to the vegetable garden in several ways. Thanks to its dense root network, it loosens the soil, improves its texture, enriches it with nitrogen, and suppresses adventive plants. It forms a plant cover that protects loamy and sandy soils from leaching during heavy rainfall.

It stores residual nitrogen in its tissues. Once mowed, shredded, and buried in the soil, it decomposes gradually, making it particularly valuable for enriching poor soils. Green manures like phacelia are primarily used in the vegetable garden, but you can also use them in poor, degraded, or overly compacted soil before planting a large bed or hedge, for example.

In soils that are not too clayey, planting phacelia allows for gardening without the need to dig. In the vegetable garden, it is ideal for preparing the soil and allows the earth to rest between crops.

It can also be used as a melliferous plant. Its nectar-rich and pollen-producing flowering occurs from spring to late summer, naturally attracting pollinators.

The flowers of Phacelia tanacetifolia attract many foraging insects

When to Sow Phacelia Seeds?

The phacelia seeds are sown from spring, starting in March until July when it is warm enough, between 10 and 25 °C. It remains in place for at least 3 months and should be mowed before flowering. You can also perform a late sowing in September. In mild regions, this autumn sowing will cover the soil throughout the winter and produce plants that will flower in spring.

How to Sow It?

This green manure requires a previously weeded soil, quickly loosened with a spade to a depth of 2 to 5 cm, and roughly raked.

• Sow by broadcasting, respecting a quantity of 1 to 2 g per m²,
• To ensure even distribution, you can mix the seeds with fine sand,
• Cover with very little soil,
• Firm down with a rake and then water gently until germination, which usually takes between 7 and 10 days.

Maintenance and cultivation of phacelia

Keep the soil moist throughout the germination period. After that, watering will only be necessary when the soil is dry, especially during dry spells.

It is not known to suffer from any diseases.

Cut the phacelia approximately four months after sowing. It is better to cut it before it goes to seed, as it can become invasive through self-seeding. Once cut, you can incorporate it into the soil through shallow digging (about 10 cm deep), add it to the compost, or leave it in place as mulch: as it breaks down, the mulch enriches the soil with organic matter, rich in nutrients. You can also shred it by mowing.

You can also allow the plants to seed in place to benefit from self-seeding or to collect seeds. Alternatively, you can simply pull it up: you will instantly have well-aerated soil, ideal for planting your vegetables.

It easily associates with other green manures such as white or purple clover.

Multiplication

Phacelia is propagated solely by sowing. Seeds harvested from the garden will retain their seed viability for about 4 years.