Anna Pavord: Planting new trees and shrubs in autumn

It may seem premature, but the fallen leaves provide sustenance, and the soil is still warm and welcoming from summer

Luma apiculata 'Glanleam Gold' can be clipped as easily as box (Alamy)

Cynics may see nothing but self-interest in a garden-centre banner urging us to plant in autumn rather than spring. Nurseries and garden centres want to spread the load, even-out the cash flow, have less stock on their hands through the potentially tricky months of winter.

But with me, they are preaching to the converted. I have always done the bulk of my tree and shrub planting in autumn. For a start, there is far too much else to do in spring. And it suits the plants. The soil is still warm from summer – especially this summer – and provides a more inviting medium for roots to explore than the dank, chilly soils of early spring.

Leaves are falling, or have fallen, so the tree or shrub does not have to slog like a steam engine to pump up enough water through the roots to satisfy the greenery above. Lack of water is the biggest single reason for death in newly-planted trees. At least a third never make it through their first summer.

As the network of leaves and shoots shuts down for the winter, trees and shrubs divert their energy to the underground root system and rapid root growth (the most rapid of the year) takes place between now and the end of the year. This means that the plant is well-established for the explosion of growth in spring.

I am particularly keen on autumn-planting for trees because it means I can buy them bare-rooted rather than container-grown. A container is a measly thing for a tree to sit in and sometimes the root system gets horribly crowded, with the main roots coiled in a heap that is difficult to straighten out at planting time.

Trees that are field-grown and lifted just before despatch have a more generous and balanced root system as they have never been pressed for space. The roots are usually trimmed after lifting to make a manageable bundle, but even so, you end up with a healthier-looking root run.

I've been thinking about all this because I've been clearing out an area of the flower garden. It's not a huge space, a roughly equilateral triangle with the base line, as it were, stretching 210cm/7ft along a retaining wall of stone, which at this point is about 80cm/2ft8in tall. The wall lies immediately on the left as you turn into the yard and is mostly covered with Spanish daisy (Erigeron karvinskianus) which I planted in one crack and which has now taken over all the others. The ground behind the wall slopes to the south-west.

For too long the area in question has been left to its own devices. The bits either side of it aren't too bad: a rosemary rescued from a London roof garden tumbles down the bank on the left-hand side, covering yards of ground and never causing a quarrel. The other side of the cleared triangle has sprawls of grey-leaved Euphorbia myrsinites, many of them self-seeded, and felted clumps of lambs' ears (Stachys byzantine) which border the path leading up through the flower garden.

Early spring looks after itself as there are masses of bulbs in the seemingly bare ground of the cleared area: crocus, 'Rip van Winkle' daffodils, species tulips, scillas. They like the fast-draining, gravelly round behind the retaining wall. There's also a self-perpetuating colony of > love-in-a-mist (nigella) that has now got too large. After that, very little. The area, I decided, could do with more bulk. The flower garden, so-called, needed shrubs, preferably evergreen.

Myrtle has dark, neat, aromatic leaves (Alamy)

My first move was to put in a myrtle, on the right, next to the low, grey spurges. There's a big one on the south-facing front of the house and, over late summer, the bees working the small white flowers made so much noise I thought there must be a swarm hiding among the foliage. I love everything about myrtle: the dark, neat, aromatic leaves, the flowers with their prominent bosses of stamens, the habit of growth which keeps the plant well-furnished right down to the ground.

Here in the flower garden I thought I'd clip the bush – either a cone or a ball, nothing complicated. In another part of the garden we've been clipping myrtle's variegated cousin Luma apiculata 'Glanleam Gold' into cones and it has responded very well. The leaves, being small, take clipping as well as box does. Neither myrtle nor luma are considered more than frost-hardy, but so far this has caused no more than an occasional hiccup. In our worst winter, some of the foliage got cut back but the damage was soon repaired.

Since planting the myrtle, I've been thinking what else to put in to provide the permanent markers that the area needs. The rosemary and myrtle together suggest a vaguely Mediterranean scenario, but I rejected cistus because it would flower while the nigella was still at its peak. One of the reasons the myrtle is so perfect is that it is at its best during August. And it is evergreen. I considered sage (yawn, yawn), artemisia (treble yawn) and abelia, a reasonably compact shrub which has many of the same qualities as myrtle. It has neat foliage (though only semi-evergreen) and it flowers from late July through to October.

Yes, I did think about abelia. But I wasn't particularly keen to bring the flowers' tinge of pink into this area, where I often plant marigolds such as 'Indian Prince' and 'Touch of Red', both tawny orange beauties which wouldn't be happy with the abelia's boudoir tones. So, as I was tunnelling away to get out (I hope) the last of the deep-rooted dandelions that had been spreading their rosettes under the drying stems of the nigella, I decided that I'd plant two more myrtles, roughly echoing the triangular shape of the cleared area, all of them to be clipped into loose cones.

It took a long time to get to what I now feel is the obvious answer. The flower garden has got far too much in it; one area guided by the 'less is more' rule will do it good. It'll be a calm, faintly-organised start to a border that ends in horticultural havoc.

WHAT TO DO

Continue to pick tomatoes, which have had a fabulous season. I've had excellent crops from 'Sungold' (of course), but also from 'Rosella' which I grew from seed sown on 28 March. Both have that balance of sweetness and acid which gives outstanding flavour. The larger-fruited 'Pannovy' (brought as young plants from Simpson's Seeds; simpsonsseeds.co.uk) has also been superb.

WHAT TO SEE

Open tomorrow (2-4.30pm) are The Watergardens, Warren Rd, Kingston-upon-Thames KT2 7LF, once part of the famous Coombe Wood nursery planted by the Veitch family in the 1860s. In these nine acres you'll find a fine Japanese garden, ponds, streams, waterfalls and magnificent trees, now just beginning to transform themselves with the colours of autumn. Admission £3.50.