The late late show: Sarah Raven reveals that August isn't just for hacking back mature growth

It's a hot old day when we stop in at the Waterlily House at Kew Gardens, where the gardener is pushing a barrow load of prunings and deadheaded blooms. "How many days' worth is that?" I wonder aloud. "Two or three," she smiles in reply. "Everything grows so fast at this time of year."

Though many plants slow down as the days begin to shorten, the August garden is all about this cutting back of mature growth. As needy as a teenager, it requires reining in, training up and copious feeding. And just like the parent of teenagers, gardeners can find themselves longing to look after babies: young things, simple, clean and new.

Fortunately, just as I was feeling all wistful about the time of year, Sarah Raven's new catalogue plopped on to my doormat with a pleasing reminder that late-summer gardening doesn't have to be all chopping and hacking. Don't imagine, she says, that you can sow seeds only in spring; 2011's late-summer-sown annuals will be stronger in 2012 than those sown so far, and will bloom earlier.

Her suggestions for annuals to sow now are extremely tasteful: to begin with, there's the delicious Cerinthe major 'Purpurascens', or honeywort. It's one of the best plants ever: a bit like a euphorbia foliage-wise, but with deep inky-blue flowers, below spectacular bracts the colour of the sea on a stormy day. They do require a tiny bit of expertise: the seeds have a hard coating, and it's a good idea to soak them in some tepid water overnight before sowing them into flower pots on a sheltered window sill. (Though they seem to happily self-seed once established, despite their lack of expertise in soaking themselves.)

Another corker Sarah Raven offers is Euphorbia oblongata, one of the very loveliest euphorbias. Once in full growth it can be cut for flower-arranging or left to add a brilliant limey tint to the flower bed. They need to have the flowers removed to keep producing blooms, and if picked, could flower for months. Best grown in a kind of dappled shade – not under a tree, but where it'll have some direct sun for an hour or two a day.

But there's one essential annual plant missing from Raven's list, as far as I'm concerned. Aquilegia was known as "Granny's bonnets" when I was little, though I can't imagine many grannies have a bonnet these days. A hoodie, maybe. Anyway, these are lovely plants, with a perky way of popping up in spring, straight-necked and prim, with pretty blue and pink flowers. Thompson & Morgan sells a wonderful selection of deep colours, "Crown Jewels Mixed", £2.99 for 20 seeds (thompson-morgan.com). These need to be sown on the surface of damp compost. Use small pots topped with a freezer bag held on with a rubber band to keep moisture in till they germinate. It can take a few months – but they're worth it.

Sarah Raven's annuals can be ordered from sarahraven.com or from the catalogue

Seeds to sow in summer

1. Nigella Damascena 'Oxford Blue'

You might think sexy chef, but these are star-like, deep midnight blue gems. Scatter a packet and you're sure to have flowers in spring. 200 seeds, £1.49, thompson-morgan.com

2. Opium Poppy 'Peony Flowered Mixed'

Horribly gaudy on the packet, but the reality is beautiful, big, blowsy flowers with cool pale green foliage. Again, you can scatter them where they will flower. 1,500 seeds, 69p, thompson-morgan.com

3. Cornflower 'Blue Diadem'

The most summery blue, you can cut a handful or leave it to blaze colour into the garden. 200 seeds, £2.19, thompson-morgan.com