August is one of the best months to sow green manures in cleared vegetable beds4. These are fast-growing plants sown to cover bare soil, and their roots and leafy growth protect and feed your ground while you plan next year's crops1.
If you've just lifted early potatoes or broad beans, don't leave that soil bare3,,4. Bare soil loses moisture quickly, and weeds move in within days4.
Why green manures matter
A green manure's foliage smothers weeds while it grows, and its roots hold the soil structure together and guard against erosion from heavy rain1.
When you later dig the plant back into the soil while it's still green, it returns nutrients to the ground and improves soil structure for whatever you grow next1.
Late summer sowing is deliberate: as summer crops finish and nutrients are left in the soil, a green manure "mops up" those nutrients before they wash away over winter1.
Leaving a bed bare invites more than weeds. Heavy summer showers can wash nutrients out of uncovered soil, and worms and other helpful soil life do better under cover than in exposed dirt4.
When to sow and when to dig in
Green manures are typically sown from late summer through early autumn, right as beds empty out1.
Most spend the winter in the ground, with digging-in usually happening the following spring, though some types can be left in place for one to two years before you turn them under1.
August is considered one of the best months for this exact job, since it gives the plants plenty of time to get established before the cold sets in4.
Green manures aren't a single plant: the botanical name is simply listed as "various," spanning annuals and herbaceous perennials1.
Most need a sunny spot to perform well, though a few tolerate a little shade, and hardiness varies: some varieties shrug off frost, others are tender and won't survive a hard winter1.
What to plant alongside your green manure beds
Not every cleared bed needs to go under a green manure. August is also a good time to sow crops like spring cabbage, kale, and calabrese directly into freed-up ground2.
Leafy, non-hearting spring cabbage types are a classic August sowing for cropping the following spring2.
Salad onions and bunching onion varieties can also go in now, giving you pickable alliums through the cooler months ahead5.
Kales, calabrese, and broccoli types started now will crop through late autumn and into winter, with some varieties cropping steadily right through to early spring2.
Carrots sown in modules or pots in August are another option for filling gaps before the season fully turns2.
Turning a "finished" bed into productive ground
The trick for late summer, according to one allotment grower, is to stop sowing for summer harvests and start sowing for autumn, winter, and next spring instead3.
A bed that gave up its broad beans or early potatoes doesn't need to sit empty until next year — a surprising number of crops will still germinate and grow before or through the colder months3.
Rather than leaving a gap, spread a layer of compost or well-rotted manure over a cleared bed, sow a green manure, or prepare it straight away for your next crop4.
A simple August routine
As each bed clears this month, work through the same short routine. Clear old crop debris and any weeds first, so the green manure or new seedlings aren't competing from day one4.
Choose green manure for beds you won't need again until spring, since digging it in properly needs a few weeks' lead time before planting1.
Choose overwintering onions, spring cabbage, or kale for beds you want cropping through winter and into next year2,,5.
Keep new seedlings watered through any remaining warm, dry spells, and watch for slug damage on young cabbage and kale transplants, since damp August evenings suit slugs well4.
By treating August as a second sowing season rather than the end of one, you keep beds working for you right through winter instead of standing empty3,,4.