Summer stock: It's time to turn our attention to the floral superstars bringing drama to the garden

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"Follow that" the garden whispers insidiously as I wander up the stone path through the dried foliage of tulips, mounds of finished peonies, and the now-naked stems of bearded iris. I can't remember any year providing quite such a spectacular display as this one has, during the months of April, May and June. Perhaps it was because winter went on so long, that spring seemed so special this season.

Spring happens, whatever a gardener does to get in its way. The next three months are trickier, in terms of matching the great waves of excitement we've already had. But there's a hydrangea moment still to come. An explosion of lilies. An agapanthus act. And the things that are good now – herbaceous geraniums for instance, won't suddenly disappear. They'll just get rangier and slightly more difficult to keep upright.

'Brookside' remains my top favourite geranium, generous with its clear blue flowers, and excellent if it can prop itself up against something sturdy, such as a mound of the big spurge Euphorbia characias, or wander through the scaffold stems of crambe. I never think of herbaceous geraniums as stars, but equally, could not do without them. 'Brookside' is easier to manage than rangy field geraniums such as 'Mrs Kendall Clarke'. It does not grow as tall (60cm as opposed to 1m) and flowers flat out from June through to August. 'Mrs C' has generally packed her bags by the end of July, though sometimes flowers later, if you remember to slash plants down to the ground after the first flush.

You need the supporting cast though. A garden can't be made only of stars. And you can usually enhance lesser plants by giving them good neighbours. I wouldn't call the smoke bush (Cotinus coggygria) a star, but you can certainly make it shine more brightly by adding a few outriders – clumps of herbaceous geraniums (magenta as well as blue), some pink-rayed lilies, a stand of tobacco flowers.

You can also lift the effect of the purple-leaved cotinus by planting it so that you see it against the light. Viewed this way, the foliage becomes translucent, like watered-down wine. Purple foliage plants can become too heavy a presence in the garden by high summer and this simple device lightens the load. You need to manage the bush, too. It can do three good things: provide a strong foliage backdrop, a haze of pale-pink smoke (the flowers, at their best this month) and vivid autumn colour, though this is better on acid soils than alkaline ones.

But you have to decide whether you want extra good foliage, or the smoke. You get the best leaves by pruning the shrub hard back every spring. But then you get no flowers because they are only produced on wood that is at least three years old. If you want both, cut out just a few branches at ground level each spring. You'll get a supply of new foliage without entirely sacrificing the halo of July smoke.

In our garden, annuals are playing a larger than usual part in seeing the garden through the next three summer months. I got carried away by the possibilities of the new greenhouse (and the cheapness of most packets of seed) and raised far too many plants. There are two problems with raising plants from seed. The first is restraining yourself from sowing the whole packet. The second is persuading yourself you do not need to prick out every last seedling. Empathy can go too far I know, but if I throw a small, underdeveloped seedling into the waste bucket, I hear a small scream. It's unnerving.

You get better plants by pricking out the seedlings, not into trays, but into individual 7cm (3in) pots. It takes up more room and uses more compost but when you transplant there's less disturbance. Plants establish quickly. So three different kinds of tobacco plant are weaving through the flower garden, and if the slugs leave them alone, they should flower until autumn. I sowed both 'Fragrant Cloud' and Nicotiana mutabilis on 14 February. 'Fragrant Cloud' grew on faster, was planted out earlier and had its first flowers just as the last of the columbines faded last month.

Neither had extra heat when germinating, but the greenhouse, having four feet of concrete block all round before the glass starts, works like a storage heater. Even when nights were cold in late February and March, the place did not cool down too sharply. Germination was excellent, with pots of seed sealed up in polythene bags set on the high shelf.

Nicotiana mutabilis gets enormous (120cm high and almost as wide) and I've used it round big mounds of Euphorbia characias. By late June, the euphorbia's green-yellow heads need cutting down, and although the leafy stems are still handsome, in terms of commanding attention the plant takes a few steps back. The tobacco flowers of N. mutabilis can take over. They are white when they first come out, but as they age, they flush from pale to dark pink, so the plant is speckled with flowers of several different colours. The effect is better than it sounds. I grew this nicotiana for the first time last year, but planted them out too close together. The rosettes of leaf spread hugely before the flowering stem starts to grow.

The tobacco flowers have been easy to grow. Even easier is the love-in-a-mist which, when autumn sown, usefully comes into flower at the end of June just as the bearded iris are finishing. Sowing in this case means nothing more than uprooting the flowered stems when the seed pods are fully ripe and shaking the seed back over the ground. Bearded iris like the sun on their rhizomes and are not good at sharing space in a border. But love-in-a-mist has light, filigree foliage under its sky-blue flowers. It is a good follow-on plant for the iris. The annuals I've planted – ammi of different kinds, orlaya, cornflowers, English marigolds, Californian poppies, sunflowers, as well as the nicotiana – may be no more than summer curlicues, temporary garnishes around the more solid dishes, but they shift the garden into a different gear. They provide the horticultural equivalent of beachwear.

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