Sowing kit: Seeds of annuals will give a patchy area in your garden some shape and colour

Like the sofas and side table in the sitting room, the flower garden at our place is furnished with some big things that mostly stay where they have been put: huge spurges, the fleshy arrow-shaped leaves of arum, fat clumps of monkshood and sea thistle. But among the set pieces, I use different annuals each year, to change the way the whole area looks.

If you've got a bitty patch of planting that does not quite hang together, a swathe of cow-parsleyish ammi or orlaya weaving in and out of the disparate elements in the group may be just the way to make it sing. If you lose perennial plants or shrubs during winter, then a sowing of annuals such as Californian poppies or marigolds will fill the bare space most beautifully, while you decide whether to replace the corpses.

You will perhaps be so enchanted by the gaiety of annual flowers that you'll never go back to the Michaelmas daisies or the viburnum. Annuals are not programmed to do anything but flower; their only imperative is to set seed before the summer ends. Then they die. Hardy annuals, such as the Californian poppy, self seed easily in soil that is light, open and well-drained. The seed usually germinates in autumn, and the plants survive through winter to flower in early summer.

Unlike most annuals, Californian poppies have beautiful foliage, grey, finely cut, mounding up in a way that's far more elegant than your average artemisia. With us, they grow best in an area covered with 6mm gravel, thickly planted with small species tulips and fritillaries. When the tulips are going over, the Californian poppies are accelerating into growth again, so they make good partners.

The wild kind are brilliant orange, but breeders have developed flowers in cream, pink and mauve, frilly doubles as well as the simpler singles. 'Peach Sorbet' (Thompson & Morgan £1.99) has double flowers of pale peach-pink. 'Carmine King' (Chiltern £1.80) is a deeper pink. If you cut stems in the morning before the buds unfurl, the flowers will last several days in a vase.

I raise most annuals in pots (you get much better plants this way), but Californian poppies have carrot-like roots, not easy to transplant successfully, so are best sown direct into the ground. There are three things to remember. First, don't sow too soon. If the soil is still cold and wet, the seed will rot. Second, rake and smooth the soil to make a welcoming bed. A small seed cannot break its way through hefty clods of clay. Third, sow thinly.

Hardy annuals are best sown from mid-April onwards, but some half-hardy annuals can be started in early March in pots under cover. The difference between the two is that seedlings of hardy annuals can usually stagger through winter. Seedlings of half-hardies (snapdragons, spidery cleomes, cosmos, tobacco plants, castor oil plants, French marigold, zinnia) cannot. None of these can be planted outside until the danger of frosty nights has passed.

All of the half-hardies I've just mentioned are superb additions to a flower garden and great for cutting, too. So if you've got a few spaces in your border, start sowing now. Sow, initially, into a 13cm/5in pot of compost that you have first watered and allowed to drain. Cover the seed with a thin layer of vermiculite. I don't use a heated propagator, but if you do, a seed catalogue will generally suggest the temperature to set.

My earliest spring sowing is usually of cleome, the spider flower, which, in the mild autumn of last season, was still looking good in November. I favour the white kind ('Helen Campbell', £1.95 for 80 seeds from Sarah Raven) which, last year, I started off on 3 March. Germination is erratic, but the seed seems to respond best to temperatures that fluctuate as widely as possible between day and night. That's why early sowings usually work better than later ones. But the plants take a long time (minimum 18 weeks) to work up to flowering size, which is another reason to give them an early start.

When the seedlings have grown to a reasonable size and are showing true leaves, you need to separate them and grow them on. It is a pleasing, peaceful job to do, provided you have time to sink into the restful, repetitive nature of the work.

Fill a series of 9cm/4in pots with compost and make a hole with your index finger in the centre of each one. Gently ease a seedling out of its original home, holding it by a leaf, not the stem. Drop the seedling into the hole, not so deep that its first leaves are buried. Firm the soil in the pot with your thumbs and, at the end of the session, water them all. Stand them out under cover and keep them watered until it is time to plant them out. This is unlikely to be before May.

Cleomes bear big, rounded heads of flower with weird spiders' legs sticking out all round. They make robust plants, 120cm/48in tall, more like perennials than annuals, and look terrific mixed with the ever-useful Verbena bonariensis or set against mounds of evergreen Euphorbia characias.

Thompson-morgan.com, 0844 573 1818; chilternseeds.co.uk, 01491 824675; sarahraven.com, 0845 092 0283