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Sowing hardy annuals in autumn

It may sound strange, but early autumn is a great time to sow flowers for next summer. The Duchy of Cornwall Nursery's Becky Martin explains why and gives her top tips on which seeds to try.

What is a hardy annual (HA on a seed packet)?

Plants which germinate, flower and set seed all in one year are called annuals (and are usually sown in spring, to flower some months later).

Starting them off in autumn gives them a massive head start: they will be bigger plants, flowering before their spring-sown counterparts, and help fill the gap between spring bulbs and summer flowering perennials.

Even if nothing happens on the surface, it’s all going on underneath

They will appear to grind to a halt over winter as day length and light intensity reduce, and temperatures fall, but beneath the soil root growth continue. They then burst into action early the following spring and make stronger, chunkier plants than those sown at the usual time. After the trials of winter they may look unpromising, but they really do pick up quickly and perform well.

It saves time!

Other advantages include that we may have more time in the garden at this time of year, our pots and seed trays are vacant, and there is the satisfaction of using seed from flowers you have grown yourself.

How to get started – in the ground

You can sow the seeds exactly the same way you would in the spring, that is either directly into the ground, or in containers under cover.

To sow them in the ground you need a bare, weed-free patch of soil with a fine crumbly tilth. This can be tricky as most of us aim to have no empty patches in our flower beds and go to some effort to fill the gaps with late flowering perennials.

If you are sowing directly into the ground, you can scatter the seeds over the prepared patch, rake the soil to work the seeds in, then firm gently and water (this is called broadcast sowing).

Alternatively you can sow the seeds in patterns which make it easier to identify them as they emerge. You can use short lines, or arcs, or a noughts-and-crosses grid. The pattern will disappear as the plants get big.

Another trick is to sprinkle sand or grit over the seeds as you sow them to mark their position. This makes it easier to distinguish and care for your seedlings as they emerge. You will be less likely to mistake them for weedlings, stand on them, or forget where they are.

The seedlings will need to be thinned out as they grow, but not ruthlessly because you will inevitably lose some over the winter to the weather and slugs, and can do a final thinning in spring. Don’t forget to label them.

How to get started – in a greenhouse

If you choose to sow your seeds in modules, pots or trays in a greenhouse you proceed exactly as you would in spring. The only difference is that as soon as they have germinated you need to remove them to somewhere cooler outside to grow on while the weather is still warm.

As soon as they are big enough, prick out the germinated seedlings to enable each one to make maximum growth. You can prick them out into small pots, or multicell trays. If they are big enough before winter sets in, the young plants can be planted into the ground outside.

Alternatively, put them in a cold frame or greenhouse, or porch, until spring. There is no hard and fast rule here; it will depend on your individual facilities and space but, as always, be guided by the packet instructions for that particular plant.

Small young plants will fare better planted in the ground before winter, than sitting in a plastic tray on the soil surface outside with no protection.

The following are some of the easiest and most rewarding plants to sow in the autumn:

  • Ammi (bihop’s weed)
  • Borage
  • Calendula (marigold)
  • Cerinthe (honeywort)
  • Centaurea (cornflower
  • Eschschsolzia (Californian poppy)
  • Nigella (love in a mist) – see pic above.
  • Orlaya

and last but not least, sweet peas, though these are best left in individual pots or tubes until planting out in spring.

Plants such as Cosmos are half-hardy annuals (HHA). They are not tough enough to cope with overwintering and need to be sown in spring.

The variegated honesty above (Lunaria annua ‘Alba Variegata’ AGM) was sown on 8 September 2020, germinated on 7 October 2020, pricked out into individual pots, overwintered in an unheated greenhouse and planted out in the garden in March 2021. The picture was taken on 2 June 2021.

duchyofcornwallnursery.co.uk

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