Clay soils are notoriously difficult for the gardener, but once improved by careful cultivation and liberal dressings of organic matter there are few plants that will not thrive. What was heavy, cold and wet becomes a very fertile growing medium.
Although clay may sometimes appear to be coarse and lumpy, the opposite is true. It is made up of very fine particles that stick together in an almost impenetrable mass. Water fills the tiny spaces between the particles, saturating the ground, in contrast to the free-draining qualities of a sandy soil made up of much coarser particles.
Evaporation from this wet mass has a cooling effect – just as one’s own body cools after a swim – so coldness as well as wetness is a characteristic of clay, with an extreme reluctance to warm up in spring. Attention to drainage may be the first need, but improvements to the soil structure then follow a well-established pattern. It should be clearly understood that clay may be either acid or alkaline, depending on the locality of the garden.
Meanwhile, anyone who acquires poor clay will wish to press on with planting trees and shrubs, which are relatively slow to mature, rather than first spending years improving the soil. Fortunately, there is a good selection of species that will tolerate cold, wet conditions, provided the soil is not continuously waterlogged.
Trees: All the following genera will cope quite well with cold, wet conditions Acer (maple); Aesculus (horse chestnut); Alnus (alder); Betula (birch); Carpinus (hornbeam); Crataegus (ornamental thorn); Fraxinus ; Ilex (holly); Laburnum; Mains (crab apple); Populus (poplar); Prunus (flowering cherry); Quercus (oak); Salix (willow); Sorbus (whitebeam), rowan and mountain ash; Tiha (lime).
Some of these are suitable only for large gardens, but many are compact enough to have a place in smaller gardens.
Alders, in particular, are closely associated with cold, moist soils, but some will not thrive in alkaline soil. Among those more tolerant to lime, making them good all-rounders, are Alnus cordata, A. glutino-sa and A. viridis. A. glutinosa ‘Imperialis’ is a particularly attractive cut-leaf form of the common alder.
Where space permits clumps of Betula pendula (common silver birch) provide a striking effect. B.p.
‘Dalecarlica’ (Swedish birch) is a particularly graceful beauty, having a pendulous habit more pronounced than the type.
Poplars are fast-growing, suckering trees suitable for clay soils. Populus x candicans ‘Aurora’ is a medium-sized tree with attractively variegated leaves. Because variegation diminishes as the tree matures, hard pruning is needed to encourage the young growths that will maintain this decorative feature.
If you have space for an oak, our two native trees, Quercuspetraea and Q. robur, will both grow in clay, but the former is to be preferred for soils that remain more or less permanently moist. An even better choice is Q. nigra (water oak), which is relatively small but needs a lime-free soil. However, it may not be so easy to obtain as the other two. Shrubs With their roots closer to the surface than those of trees, shrubs are likely to benefit somewhat sooner from improvements made to the condition of the soil. Mulching is valuable, for it will help to reduce the degree of shrinking and cracking as the soil dries out.
Best shrubs to grow in clay soils
Suitable shrubs may be divided roughly into two – those (Group A) suitable for growing in unimproved clay and others (Group B) that may be chosen when conditions improve or if given a good start in individual planting holes liberally supplied with compost.
Group A First-stage shrubs include
Aucuba; Berberis (barberry); Diervilla; Forsythia; Kerria; Philadelphus (mock orange); Pyracantha; Ribes (flowering cur rant); Spiraea; Symphoricarpos; Viburnum.
The following are especially worth considering:
Of the several named forms of Aucuba japonica, a shrub often planted in difficult situations, A.j. ‘Nana Rotundifolia’, is a small female version that berries freely.
The various species of Berberis, both evergreen and deciduous, will do well provided the soil is not waterlogged. A particular favourite is B. darwimi, an evergreen that displays masses of deep orange-yellow flowers in April and May.
Colutea arborescens (bladder senna) derives its common name from its inflated seedpods. Though rather large, growing to 4 m (13 ft), it may be pruned hard in late winter. Alternatively, if you are able to buy C.a. ‘Bullata’, this will prove denser and will grow more slowly.
There is more choice in forsythias than many realize.
‘Beatrix Farrand’, ‘Karl Sax’ and ‘Lynwood’ have canary-yellow flowers, those of the first-named measuring up to 2.5 cm (1 in) across.
‘Arnold Dwarf has a fairly compact habit, with a height and spread of about 90 cm X 1.8 m (3 ft x 6 ft). However, the rather sparse flowers are not particularly striking and the plant is sometimes used as ground cover.
Ribes sanguineum, a widely grown flowering currant, has several named clones. Two of the most popular are ‘King Edward VII’, with deep crimson flowers, and ‘Pulborough Scarlet’, again with deep red flowers. Also worth considering are ‘Album’, with white flowers, ‘Flore Plcno’, with double rosy-red flowers, and the lovely ‘Brockiebankii’, with pink flowers and golden-yellow leaves.
Depending on the variety grown, viburnums offer winter flowers, autumn colour and ornamental fruits. To allow full enjoyment of the flowers, remember to plant the shrubs where they are visible from the house.
Viburnum x bodnantense and V. x b. ‘Dawn’ flower between late autumn and late winter, the white blooms being delightfully scented. During April and May, the scent of V. carlesii is reminiscent of Daphne. Two particularly striking forms of V. plicatum are ‘Lanarth’ and ‘Mariesn’, their widely spreading branches, giving a layered effect, displaying heads of white flowers in early summer. Group B Three shrubs that will grow in clay soils made really free-draining are camellias, magnolias and rhododendrons.
Of the camellias, C. japonica and C. x williamsii have produced numerous named cultivars. The simplest way to choose is to compare young, containerized plants while they are in flower.
Magnolia x soulangiana also flowers while the plant is still young and it is one of the best of its genus for growing on a clay soil. There are named forms, of which ‘Lennei’, with late spring flowers that are rosy-purple outside and whitish inside, is a good choice. A sport of this, ‘Rustica Rubra’, is rich rosy-red. M. stellata is a good choice for small gardens.