Why the Ground Beneath a Tree Is Hard Work

The soil under a mature tree is one of the toughest planting spots in a British garden. Rain barely reaches it — the canopy intercepts it, and what does fall is often funnelled away by the drip line. Meanwhile the tree's roots, many of them fine and hungry, fill the top 30cm of soil and take the moisture and nutrients before anything else gets a look in. Add a thick layer of leaf litter and you have a dry, dark, competitive patch that defeats most bedding plants by August.

So the aim is not to fight the tree. It is to choose plants that expect these conditions, to sneak them into gaps between the roots, and to nurse them through the first two summers. Done well, a bare circle of earth beneath an oak or a birch can become the most interesting corner of the garden.

Plants That Actually Cope: Geraniums, Epimediums and Ferns

These three groups earn their place over and over in British gardens because they evolved on woodland edges where trees take the water and the light.

• Geranium macrorrhizum is the workhorse of dry shade. Its aromatic, semi-evergreen leaves knit into a weed-suppressing carpet, and it flowers in early summer in pink or white. 'Bevan's Variety' and 'Ingwersen's Variety' are both reliably tough.
• Geranium phaeum suits damper shade better. The dusky purple or white flowers are quieter, but the foliage stays neat and the plant seeds itself politely into gaps.
• Epimediums are the connoisseur's choice for the base of a tree. New foliage arrives in spring washed with bronze and red, and the small flowers hover above the leaves. Epimedium x perralchicum and Epimedium x versicolor 'Sulphureum' both tolerate deep, dry shade once they have settled, though they need watering while their roots take hold.
• Dryopteris filix-mas, the male fern, and Dryopteris erythrosora, with its coppery new fronds, are as tough as they come. Polystichum aculeatum and Polystichum setiferum stay evergreen and look good even in January.
• Luzula sylvatica, the greater woodrush, is excellent under trees where little else will grow, forming dark green tussocks that shrug off drought.

Where the shade is damp rather than dry, you can add hart's tongue fern, Brunnera and Pulmonaria. Under deciduous trees, plant spring bulbs and wood anemones too — they flower before the canopy closes and then quietly disappear.

Planting in Small Pockets Between the Roots

You are not going to dig a generous border under an established tree, and you should not try. Cutting through substantial roots can harm the tree, and if it carries a Tree Preservation Order or stands in a conservation area you may need permission before doing any work near it. Work with the roots, not through them.

Use a hand trowel rather than a spade and feel your way around the roots, easing small plants into whatever gaps you find. A 9cm pot or a bare-root plant will slot in far more easily than a bulky container. Plant away from the trunk itself, which is the driest and most root-congested area.

• Plant in autumn if you can, when the soil is still warm and moist and roots can grow on before winter.
• Mix a couple of handfuls of garden compost or leafmould into each pocket, but do not over-enrich the soil — too much fertility simply feeds the tree's roots.
• Firm gently around the root ball and water immediately, even if rain is forecast.
• Group plants in loose drifts rather than as single specimens, so they shade each other's roots.
• Mulch afterwards with 5cm of leafmould or chipped bark, keeping it well clear of stems and the trunk.

Watering to Get Roots Established

The first summer decides whether your planting succeeds. Tree roots will out-compete a new perennial for every drop, so you need to water deliberately rather than hopefully.

Give each plant a full watering can — four to five litres — twice a week for the first six to eight weeks after planting, then weekly through any dry spell for the rest of that first season. Water slowly, at the base, in the evening, so it soaks in rather than running off the surface. A little and often is worse than useless: it encourages shallow rooting.

A short length of pipe or an upturned plastic bottle with the base cut off, sunk beside the root ball, will get water down past the tree's surface roots to where it is needed. On a larger scheme, a soaker hose laid through the bed and run for half an hour twice a week saves a great deal of carrying and delivers water more evenly.

By the second summer, most plants will manage with a good soak once a fortnight during prolonged dry weather. By the third, the toughest of them — the geraniums, epimediums, woodrush and shield ferns — should be looking after themselves.

Long-Term Care and a Few Extra Companions

Keep the leaf litter. It is free mulch, weed suppressant and soil improver in one. The only caution is that a thick, wet blanket of leaves in November can smother small plants, so rake it off low crowns and let it lie around them instead.

Lift and divide geraniums and epimediums every three or four years in early spring if they start to thin in the middle. Cut back the old fronds of deciduous ferns in late winter, just as the new croziers unfurl, and leave evergreen ferns alone apart from a tidy-up. Dead leaves on epimediums are best trimmed in late winter too, before the flowers push through.

Once the ground cover is working, ring the changes with cyclamen, snowdrops and hellebores, and in a more generous soil try a Japanese painted fern. Planted in pockets, watered patiently and left to knit together, these are the plants that make the space beneath a tree feel like woodland rather than a wasteland.