What to plant in September

September is the month the plot quietly empties and most people let it. The summer crops are finishing, the light is going, and there is a strong temptation to tidy up, cover everything over and call it a year.

That is the expensive decision. The soil in September is still warm - warmer than it will be in April - and seed germinates fast in it. What it is losing is daylight, and that is the clock you are actually racing. Every week you delay in September costs you roughly a fortnight of growth at the other end, because a plant that goes into winter small stays small until March.

Sow now and you eat in the hungry gap, that stretch from February to April when the stored crops have run out and nothing new is ready. Leave it and you buy salad in a bag all winter.

What can I still sow in September?

Winter salads, spring cabbage, overwintering onions, spinach, and the hardy oriental leaves. Almost everything you sow this month is for eating between November and April, not for eating now.

Winter lettuce goes in during the first half of the month. Sow on the surface, barely covered, thin to 20 to 25cm. Use the hardy types, not the summer ones (RHS lettuce guide). Winter Density and Arctic King will stand outside in most of the UK and crop into spring under a cloche. Sown now they establish while the soil is warm, which is the entire point.

Corn salad and land cress are the two crops that will still be feeding you in January. Both are properly hardy and both are slow, which is exactly why they need sowing now rather than in October. Corn salad 1cm deep, thinned to 10cm. Land cress tastes like watercress, self-seeds happily, and shrugs off frost.

Winter spinach is a September crop, not an autumn one. Medania is bred for it. 2 to 3cm deep, thin to 15cm. It puts on growth through September and October, sits out the worst of the winter, then grows away hard in February when nothing else is moving (RHS spinach guide).

Perpetual spinach and chard can still go in during the first week or two. 2cm deep, 30cm spacing. A single September sowing crops through autumn, survives the winter, and produces again in spring - eight months of leaves from one row.

Oriental leaves - mizuna, mustards, pak choi, komatsuna - are at their best sown now. They bolt if you sow them in spring because lengthening days push them to seed. Sow them as the days shorten and they behave. Scatter thinly and cut as baby leaves in four to five weeks, or space at 20 to 30cm for full heads.

Spring cabbage, if you did not sow it in August, can still go in during the first week of September in the south. Transplant at 30cm in October and you cut greens in March and April. Later than that and the plants will not be big enough to face the winter (RHS cabbage guide).

Overwintering onion sets go in from mid-September. Radar and Senshyu Yellow are the reliable ones. Push them in so the tip is just showing, 10cm apart, and they establish before the cold, then give you a harvest in June - weeks before spring-planted sets are ready (RHS onion guide).

Turnips and radishes sown in the first half of the month still make a crop, and they are the fastest way to fill a bed that has just come empty.

Is it too late to sow salad outdoors in September?

No, but it is the last month where it is easy, and the soil temperature is what decides it rather than the date on the calendar.

Most salad leaves want soil at 10C or more to germinate reliably. In early September the soil across most of England is still well above that. By mid-October it is marginal, and by November germination is slow and patchy wherever you are.

The practical rule: sow salad outdoors up to about the third week of September, and after that sow it under a cloche, in a cold frame, or in modules somewhere sheltered. The seed does not know what month it is. It only knows how warm the ground is.

What do I do with beds coming empty in September?

Do not leave them bare. A bare bed over winter loses nutrients to rain, compacts under the weather, and grows weeds anyway.

Three options, in order of value:

• Sow something. Winter salads, spinach, or an oriental leaf mix. Even a bed you only cut twice has earned its keep.

• Sow a green manure. Field beans, grazing rye or phacelia sown now will cover the soil, hold onto nitrogen that would otherwise wash away, and be cut down in spring. Grazing rye is the toughest and will germinate in cold soil right into October.

• Mulch it. If you have nothing to sow, put 5cm of compost on the surface and leave it. The worms take it down over winter and you sow into a bed that is ready in March (RHS mulching guide).

What you should not do is dig it over and leave it rough. That advice is decades old, it destroys the structure the worms have spent all summer building, and it leaves bare soil exposed to every rainstorm between now and March.

When should I plant garlic in the UK?

From late September through to November, and earlier is better than later in the north.

Garlic needs a cold period - roughly a month below 10C - to split into cloves at all. Plant it in spring and you get one fat round instead of a bulb. That is why it goes in now (RHS garlic guide).

Split the bulb into cloves just before planting, not weeks before. Push each clove in pointed end up, 2 to 3cm below the surface, 15cm apart with 30cm between rows. Solent Wight is the reliable softneck for most of the UK and stores until spring. Do not use supermarket garlic - it is usually a warm-climate variety and it will sulk.

What should I be harvesting in September?

Almost everything at once, which is the other reason people stop sowing.

Maincrop potatoes come out once the haulm has died back. Lift on a dry day, leave them on the surface for a couple of hours to dry, and store only the perfect ones - one damaged tuber will rot a whole sack (RHS potato guide).

Onions and shallots should be lifted and dried thoroughly before storing. If the necks are still thick and green they will not keep.

Squash and pumpkins want cutting before the first frost, with a good length of stalk, and curing somewhere warm for a fortnight. Cured properly they keep until February; picked wet and stored damp they rot by Christmas.

Runner and French beans are finishing. Leave the roots in the ground - they carry nitrogen the next crop will use.

Tomatoes outdoors are racing the blight. If the plants go, pick everything green and ripen it indoors.

Apples and pears are ready when a gentle lift and twist releases the fruit. If you have to pull, it is not ready.

What is too late to start in September?

Be honest about these rather than wasting seed:

• Carrots, beetroot, parsnips - no time to bulk up before the light goes

• French and runner beans - the first frost kills them long before they crop

• Courgettes, squash, sweetcorn - gone, and gone since June

• Brassicas for winter - kale, sprouts and purple sprouting all needed sowing in June and July

• Leeks - too late to sow, though plants raised earlier can still go in

• Broad beans - wait. Aquadulce Claudia is sown in late October and November, not now, or it grows too soft and the frost takes it

September sowing reference table

Depths and spacings based on RHS vegetable sowing recommendations.

Where should I start if I only have one afternoon in September?

Four jobs, in this order:

• Sow a row of winter salad - corn salad, mizuna and land cress. Cutting from November right through to March, and they need almost nothing from you.

• Sow winter spinach - one row now feeds you in February when there is nothing else.

• Get garlic in - it wants the cold, and it is the single easiest crop on the plot.

• Cover every bare bed - compost, or a green manure. Never leave soil naked over a British winter.

That is one afternoon, and it is the difference between a plot that feeds you until April and a plot that sits under a tarpaulin until spring.

September is not the end of the growing year. It is the start of the half of it that most people never see.