What to sow and plant in September

Ask most people when the gardening year starts and they will say spring. The soil disagrees.

By September the ground has had all summer to warm through, and it holds that heat far longer than the air above it does. Add the rain that starts to return, and the heavy morning dew that forms when warm days run into cool nights, and there is more moisture and more warmth down where roots live than at any point since May.

There is a catch, and it is the light.

Until the autumn equinox, around the 22nd or 23rd of the month, our days are still longer than our nights. That is enough light to germinate seed while the air is still warm. After it the balance tips, growth slows, and everything begins hunkering down for winter. So September is really two jobs running at different speeds. Sowing belongs to the first half of the month. Planting can carry on well past it, and the ground stays workable for weeks yet.

At a glance

• Sow in the first half: quick catch crops, and hardy things that can overwinter as seedlings
• Sow for next summer: hardy annuals, which flower earlier and longer than a spring sowing
• Plant all month: trees, shrubs, fruit, perennials and spring bulbs, provided they are hardy
• Plant bulbs at least three times their own height deep. Shallow is the mistake that costs you next year
• The thing that decides it all: warm soil, falling light

• What can I still sow this month?
• What can I plant in the ground now?
• Is it hardy, half hardy or tender?
• Why does planting now beat planting in spring?
• How deep should I plant spring bulbs?
• What kit do I actually need?
• Frequently asked questions

What can I still sow this month?

Only two kinds of thing belong in a September seed tray, and it is better to know that before buying packets.

The first is a quick catch crop, sown and eaten before the cold arrives. The second is anything hardy enough to sit out the winter as a small seedling, wanting at most a cloche or a sheet of horticultural fleece over it on the worst nights. Everything else will germinate, look promising for a fortnight, then rot or run out of light.

Vegetables

For the first half of the month, quick croppers: radish, rocket, mizuna, mibuna, pak choi and the other oriental leaves. Most are cut-and-come-again, and in a kind autumn they keep going well into November.

For overwintering, sow spring onions such as 'White Lisbon', a winter lettuce like 'Arctic King' or 'Winter Density', corn salad, land cress, perpetual spinach and chard. These stay small through the cold and put on size the moment the light returns, which is how they beat a spring sowing to the plate by several weeks.

Broad beans and peas are the ones we tend to reach for too early. They want late October or November. Sown now they make too much soft, sappy growth and go into winter oversized and vulnerable.

Hardy annual flowers

This is the sowing most gardens miss, and it has the biggest payoff of anything on this page. A hardy annual sown in September spends autumn and winter making roots, then flowers earlier, taller and for considerably longer than the same seed sown in March.

Calendula, cornflower, nigella, larkspur, Ammi majus, Orlaya, poached egg plant and opium poppies all take autumn sowing well. Sweet peas do too, in deep pots or root trainers, which gives them the long root run they want before spring.

If the garden is heavy or wet, sow into trays or modules rather than open ground, and keep them somewhere cold but sheltered. A cold frame is ideal. An unheated greenhouse, or a sheltered spot against a wall, will do the job. Our seed range is organic and open-pollinated, which matters more for an overwintering sowing than a spring one, because you are asking a plant to sit through months of wet and cold.

Hardy perennials

Aquilegia, lupin, delphinium, verbascum, achillea and primula can all be sown now. Several of these actually want a cold spell before they will germinate properly, so an autumn sowing left outside over winter does for free what a spell in the fridge would otherwise have to do.

One distinction catches people out every year. Biennials such as wallflowers, sweet William and foxgloves are not sown in September. They were sown in early summer. September is when they get planted out into the positions where they will flower.

What can I plant in the ground now?

Far more than we can sow, and this is where the month really earns itself.

Trees, shrubs, fruit bushes and canes, herbaceous perennials and every kind of spring flowering bulb can go in now, into open ground or into large containers of a decent compost mix. One rule governs the whole list, and it is hardiness.

Is it hardy, half hardy or tender?

Working definitions, which matter more in September than at any other planting time:

• Tender: damaged below about 5°C and killed outright by frost. Pelargoniums, dahlias in leaf, most houseplants
• Half hardy: takes a light touch of frost, roughly 0°C down to -5°C, but not a hard or prolonged one
• Hardy: comes through -5°C comfortably, and most hardy plants through -15°C or lower

The RHS publishes this as a formal scale, H1 to H7, and good nurseries print the rating on the label. Anything half hardy or tender put out now will not merely be cold. It will sit in ground that is wet as well as cold, and that combination accounts for far more losses over a British winter than low temperature on its own.

Why does planting now beat planting in spring?

This is the part that is easy to miss, because from above ground nothing appears to be happening at all.

When the air cools, shoots and leaves stop growing. Roots do not. They carry on extending through warm, moist soil for weeks after the top of the plant has apparently shut down, and they keep moving in every mild spell right through winter. A shrub planted this month spends the next six months quietly building the root system it is going to need.

Come spring it starts from strength. The same shrub planted in May has to make roots and leaves at once, in soil that is drying out, which is why it needs watering all summer and the September one largely looks after itself.

How deep should I plant spring bulbs?

Spring bulbs are the best value in the garden, and the reason is that somebody else has already done the difficult part. Next spring's flower bud is already formed inside the bulb when you buy it. That is why a bag of daffodils will flower even if they are planted badly.

Which is exactly the trap.

Plant at a minimum of three times the height of the bulb. For a large daffodil that means roughly 15cm (6in.) of soil over the top of it; for a crocus, nearer 8cm (3in.). Too deep is a far smaller mistake than too shallow.

A shallow-planted bulb still flowers in its first spring, because that bud was made before it ever reached the garden. What it cannot do is build the next one. Near the surface it bakes and dries in summer and never gathers the reserves to form another flower, so the second spring brings a tuft of leaves and nothing more. The bulbs get blamed. The depth was the problem.

Tulips are the exception on timing rather than on depth. They go in around November, once the ground has cooled, because planting into warm soil invites the fungal trouble that flattens a display.

What kit do I actually need?

For sowing: trays and pots, something to fill them with, and labels. Coir fibre mixed with a good quality loam makes an excellent seed compost, holding moisture without going to sludge, and a block of coir stores dry in the shed until the day it is wanted. Labels are not optional. Nothing sown in September is identifiable by February, and every gardener who has skipped that step has regretted it by March.

For planting: gloves, a hand fork and a trowel. The hand fork for lifting and dividing perennials and teasing out roots that have circled the inside of a pot, the trowel for bulbs and small plants, and gloves because autumn planting means wet soil, cold hands, and a great deal more of it than the sowing does. There is more in our garden tools and gardening and growing ranges.

If you sowed in the drought weeks and it came to nothing, the reasons are worth reading before trying again: is it worth sowing vegetable seeds in a drought.

Do the sowing in the first fortnight, do the planting whenever the ground is workable, and by the time the garden looks as though nothing at all is going on, most of next year's work will already be done, out of sight, underground.

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Frequently asked questions

Is it too late to sow seeds in September?

Not for the right ones, and where you are in the month matters more than the month itself. Two kinds of thing work: quick catch crops such as radish, rocket and oriental leaves, sown in the first half and eaten before the cold; and hardy things that will sit out the winter as small seedlings, such as spring onions, winter lettuce, corn salad and chard. What has gone is the window for anything needing heat and long days. After the equinox, around the 22nd or 23rd, the nights are longer than the days and germination slows sharply.

How deep should I plant spring bulbs?

At least three times the height of the bulb. For a large daffodil that is roughly 15cm (6in.) of soil over the top, and for a crocus nearer 8cm (3in.). Too deep is a much smaller mistake than too shallow. A shallow-planted bulb still flowers in its first spring, because the flower bud was formed by the grower before you bought it, but it cannot build another. Near the surface it bakes and dries over the summer, and the second spring brings leaves and no flower.

Why is autumn a better time to plant than spring?

Because roots carry on growing when shoots stop. Once the air cools, top growth shuts down, but roots keep extending through soil that is still warm and moist, and they continue in every mild spell through winter. A shrub planted in September has six months to build a root system before it has to support any leaves. The same shrub planted in May has to do both at once, in soil that is drying out, which is why it needs watering all summer and the autumn-planted one largely does not.

What does hardy actually mean?

As a working rule: tender plants are damaged below about 5°C and killed by frost; half hardy plants take a light frost, roughly 0°C down to -5°C, but not a hard or prolonged one; hardy plants come through -5°C comfortably and most through -15°C or lower. The RHS publishes a formal scale from H1 to H7 and good nurseries print the rating on the label. In autumn the temperature is only half the problem, because ground that is wet as well as cold kills more plants over a British winter than cold alone.

Can I sow flower seeds in September?

Hardy annuals are the best sowing of the month and the one most gardens skip. Calendula, cornflower, nigella, larkspur, Ammi majus and poached egg plant sown now spend autumn and winter making roots, then flower earlier, taller and for longer than the same seed sown in March. Sweet peas do the same in deep pots or root trainers. Hardy perennials such as aquilegia, lupin and delphinium can also go in, and several of them need a cold spell before they will germinate at all.