Sow green manure as soon as a suitable bed becomes free and conditions allow reliable germination. Late summer generally offers more warmth and growing time than late autumn. Mustard and phacelia can establish fairly quickly, but even fast-growing plants need moisture, light and enough time before cold weather slows growth.
The sowing window begins with the previous harvest
Plan from the day you remove the main crop, not from an arbitrary date in a calendar. A bed cleared in July can support a broader choice of species than one freed much later. Slower-growing legumes and complex mixes often require earlier establishment. A quick-growing green manure may still be useful later if the soil is warm and moisture is adequate.
In Britain the growing season varies between milder coastal districts, southern gardens and colder upland sites. A fixed last sowing date is therefore a rough guide only. Check the species and variety on the seed packet, recent temperatures and the local weather outlook.
Mustard as a green manure: when does it make sense?
White mustard germinates readily in suitable conditions and can make useful cover while the soil is warm. It has less time to develop when sown close to winter. In many gardens frost may kill the growth, leaving residue on the surface, but this depends on conditions and the type grown.
Mustard belongs to the brassica family. Avoid using it routinely in a rotation that already includes cabbages, kale, broccoli or other brassicas. A different cover crop may be more appropriate where crop-family disease and pest cycles are a concern.
Phacelia: useful, but not a way around late sowing
Phacelia is a common option for a quickly established cover crop and is not a brassica. Its fine foliage and rooting can help cover bare ground, but the seed still needs a workable seedbed and adequate moisture. Very late sowing cannot be made equally productive just by adding more seed.
Should you wait for rain during a dry spell?
Seed in dry soil may remain inactive until rain arrives. Lightly preparing the seedbed and sowing ahead of reliable moisture can work, but scattered seed left on a baked surface may be lost to birds or wash away during a downpour. Do not dig or rotavate saturated soil in an attempt to catch a sowing date.
Do mixes always outperform a single species?
Not necessarily. A mix works when its components can grow in the same sowing window and serve clear purposes, such as ground cover, rooting or nitrogen capture. Random mixtures can include plants that compete poorly or do not suit the next crop. Seed size also matters: different species may be difficult to distribute evenly with the same equipment.
Choose a cover crop that fits your rotation
Look at what was grown before and what you intend to plant next. Avoid repeating the same crop family, and consider whether the green manure will overwinter, die in frost or need cutting and incorporation. A living cover crop may also delay spring bed preparation if you leave it too long.
Farming scheme rules are a separate question
A seed mix chosen for an agricultural stewardship or land management agreement may be subject to particular dates, species lists and retention rules. Those administrative requirements are not automatically the biological optimum for every field. Check the current rules for your location and agreement instead of applying general garden advice to a payment claim.
For ordinary garden beds, the practical principle is simple: early, well-established cover is usually more valuable than a nominal sowing performed too late for meaningful growth. If the soil is already cold, keep it protected with suitable mulch rather than insisting on seed that may not establish.