Phacelia and other green manures for empty winter beds
Short answer
Green manure is a crop grown to protect and feed the soil rather than to eat. Sow phacelia on empty beds until mid-September, let it cover the ground over winter, then cut it down and dig it in at least four weeks before you plant in spring.
Phacelia sowing calendar
From the VegLog crop library. Change the frost dates to match your garden.
Last frost around 20 April, first frost around 31 October.
- Sow outdoors
- late March to mid-September
- Harvest
- mid-May to late October
Green manure is a crop you grow for the soil, not the kitchen. Sown on beds that would otherwise sit empty over winter, it holds the soil together, stops rain washing nutrients away and smothers weeds. In spring you cut it down and dig it in, and it rots down to feed your next crop.
Why bother?
Bare soil in winter is at the mercy of the weather. Heavy rain washes nitrogen and other nutrients down out of reach, compacts the surface and lets weeds take hold. A green manure covers the ground, and its roots open up heavy soil.
Sowing phacelia
Phacelia is one of the easiest green manures. It’s fast, its fern-like leaves cover the ground well, and if any flowers, the blue blooms are loved by bees.
| What | Detail |
|---|---|
| When to sow | Late March to mid-September |
| How | Scatter thinly over raked soil and rake in lightly, about 0.5 cm deep |
| Spacing | About 10 cm apart if sown in rows |
| Covers the ground in | Around six to eight weeks |
| Hardiness | To about -7 °C |
For winter cover, sow from August to mid-September as soon as a summer crop comes out. Phacelia isn’t related to any vegetable, so it fits into any crop rotation without carrying over pests or diseases.
Cutting it down
In late winter or early spring, before it sets seed, cut it down. Chop it with a spade or shears, leave it to wilt for a day or two, then dig it into the top layer of soil. If you’d rather not dig, leave it on the surface and cover with black plastic until it rots.
Either way, wait at least four weeks before sowing or planting. The rotting leaves can hold back seedlings for a while.
Other green manures
- Grazing rye is the hardiest, sown until late October. It’s very good on heavy soil but takes effort to dig in.
- Field beans add nitrogen and can be sown into November.
- Crimson clover adds nitrogen and suits lighter soils, sown by late August.
If it’s too late to sow anything, a thick mulch of compost or manure does a similar job.
For a full calendar of what goes where, see crop rotation for beginners, and what to sow in August for the best time to start.
Questions people also ask
When is it too late to sow green manure?
Phacelia needs sowing by mid-September in most of the UK to establish before winter. After that, grazing rye or field beans are hardier, or simply mulch the bed with compost.
Do I have to dig green manure in?
No. You can cut it down and leave it on the surface as a mulch, or cover it with black plastic for a few weeks until it rots. Either way, give it at least four weeks before planting.
Is phacelia hardy enough for winter?
It's hardy to about -7 °C, so it usually survives a mild winter in the south. In colder areas it may die back, but the roots and dead foliage still protect the soil.
Dates set to your own frost dates
VegLog works out sowing and planting times for your town, then reminds you when each job is due.
