Branching out: If you're rooting to plant new trees, now is the time to start digging

With a jigsaw puzzle, the last pieces are the easy ones. You can immediately see what fits where. You might expect it to be the same with a garden but there, fitting in the last plants is much more difficult than sorting out the first ones. I've a few spaces left for a small tree or a big shrub, but at this stage of garden-making (we've been here four years), I'm hemmed in by all kinds of constraints: the newcomers have to match the living conditions on offer; they have to fit in with their neighbours; and they have to deliver a particular gift to the garden, something that's presently missing in that place. I need a shrub that performs in late summer to fill a gap on the bottom bank, and a smallish, thinnish tree for the top corner of the flower garden, something that won't tangle with the rampageous perennials already growing there.

I started planting the flower garden, a south-facing slope beyond the yard, before I realised it would be a good idea to have a path running along the back of it. The path would separate the garden from the boundary hedge, a country mixture of holly and hazel, sloe and hawthorn. It would also allow me and a wheelbarrow to get into the border from the back.

So, rather late in the day, I marked out a four-foot path and planted box all along it. From the bottom of the bank, the border is backed by these two lines – the low, evergreen box hedge in front, with the much taller mixed hedge behind (you don't see the path between them). But the addition of this new box hedge meant that the purple-leaved Cercis candensis 'Forest Pansy' I'd planted as the beacon in the top left-hand corner of the bank, ended up too close to the neighbouring box which has amazingly wide ranging and greedy roots. The cercis hasn't like it and is now gasping. This autumn it has to move.

But what can go there instead? Even if I plant whatever it is further away from the box, this is still a thirsty spot, at the top of quite a steeply sloping bank, in full sun. I chose the cercis for its foliage, an exceptionally lovely plum purple. The leaves are rounded, held out from the branches on long stalks so that they move well. The bank, being then mostly a frippery, flowery sort of place, needed big anchoring rocks of foliage.

Euphorbias provide this almost better than anything else and on the slope there are two mounds of Euphorbia mellifera, sprouting back now after their winter shock. Of all things in the garden, they suffered the worst. Mop-headed bay trees march along the back of the border, just inside the box hedge and there are four pillars of Irish yew zig-zagging up the slope between the flowers. None of these were in place when I first planted the cercis.

So do I still need to think of foliage before anything else in choosing a replacement for the cercis? I don't think I do. What with Euphorbia wulfenii (a brilliant foliage plant in pale grey-green) and the handsome, jagged leaves of tree peony there is now more bulk on the bank than there used to be. But it would be good to find a kind of lighthouse to shine out from that top corner – something that could provide more than one treat. Fruit as well as flowers? Autumn colour as well as spring blossom?

The most important thing to address though is the site: hot and dry. I can't think Mediterranean because of an uneasy niggle that we may soon have another winter as cutting as this last one, which carved great chunks out of the bay trees as well as battering the euphorbias. A myrtle passed through my mind. We've got several already, and I love them, but this position is perhaps too exposed. The evergreen foliage, wonderfully aromatic, would be browned and burnt by winter winds.

I keep coming back to the idea of a pear, not the silver-leaved Pyrus salicifolia 'Pendula' which would take up too much room with its weeping, but something like the snow pear, Pyrus nivalis which has equally silver foliage, but a much better outline for the position I'm thinking of. The blossom is prettier too, covering the tree with white in April. Even at maturity, the snow pear wouldn't be too tall and the branches turn upwards naturally, like a candelabra, so the outline is narrow. The fruit isn't particularly showy, but it'll provide extra rations for the birds (and may keep them for a while off the holly trees in the hedge behind, always stripped before Christmas).

The snow pear is native in southern Europe, so it will be used to heat. Pears, anyway, are relatively tolerant of drought and I'll make sure this tree is ordered and delivered in November. It's far better with most trees and shrubs to plant in late autumn rather than in spring. Roots have time to settle and organise themselves before they are required to service the leaves above. Spring-planted trees have too little time to find their feet before summer heat starts to scorch them. An autumn-planted tree still needs care through the following summer, but it is far less likely to give up altogether.

This autumn, too, I can add the two final Irish yews to the flower garden. A professional garden designer would have put in all six at once, but at £38.85 each, I can't afford to do that. I know exactly where they need to go, though, and have kept spaces open for them. The idea came from a picture of the garden designer Norah Lindsay's Edwardian garden, tall thin pillars of evergreen rising from a froth of aquilegias and iris.

It'll be important not to let them get too thick or too tall. Clipping, when it's necessary, will not be easy on the slope. For the moment, their girth is nicely contained with fine fishing line, tied at intervals round the greenery. This stops the upright branches flopping out, especially in snow. If they hadn't had these corsets, they would have been ruined by the late March snow we had here, incredibly heavy, wet stuff, which tore all the branches off the poor Magnolia delavayi and caused all kinds of mayhem, especially with evergreens.

So that's three bits of the jigsaw filled in, which leaves me free to think about the rest of the gaps. And to order the pear from Thornhayes Nursery, St Andrews Wood, Dulford, Cullompton, Devon EX15 DF, 01884 266746, thornhayes-nursery.co.uk.