Chalk gardens have soil which is alkaline, ie with a pH above 7. In the UK these are found in areas such as the South Downs, Chiltern Hills and Salisbury Plain. And in areas with limestone deposits, the Yorkshire Dales being a prime example, the soil is also likely to be alkaline.
Alkaline soils often have a whitish colour and often contain pieces of chalk, lime, flints. As well as carrying out a pH test, if there is an educational aspect to the gardeners’ work, it can be fun to have a demonstration where some of the soil is added to a jar of vinegar. It will froth up, due to the release of carbon dioxide.
Gardening with Chalk
Chalky soils are alkaline and free-draining, and the depth of soil over chalk varies considerably. Limestone soils have similar characteristics. Although theoretically, the soil could have a pH of 7.1 – 14, in the UK alkaline soils are unlikely to be naturally higher than pH 9.
Chalk gardens may have light or heavy soil, depending on factors such as how stony it is.
Disadvantages
- Frequently stony
- Rapid drainage can lead to a dry soil, especially in warmer weather
- Minerals such as iron and manganese will quickly leach out of the soil if it is overly free draining
- The soil depth may be shallow
- Deep or sticky chalk soils can resemble clay soil; for example, cold to warm up, difficult to dig
- Chlorosis
Advantages
- Leaching of minerals can be remedied to an extent by regularly adding organic fertiliser
- Adding plenty of organic matter on a regular basis turns chalk gardens into fertile sites whether they are shallow or sticky
- Generally, chalk gardens have light soil, easy to dig at anytime of the year
Where there is sufficient soil depth, then the regular addition of organic matter is important. This may be compost made on site, or brought in.
- Leaf mould can be particularly beneficial where a slight reduction in the soil pH is needed, although it is not a ‘quick fix’
- Adding organic matter will improve the water retentive properties of the soil
Chlorosis, where iron is locked up in the soil and unavailable to the plants, can happen even when lime tolerant plants are grown. The annual application of sequestered iron in the spring may be required if yellowing of foliage is observed.
Shallow alkaline soils
Where the soil depth is very shallow, there are three main options
Firstly importing large quantities of topsoil. Due to the cost (of both materials and labour) this may only be an option for certain areas of the garden. Where new trees are to be brought in, it is worth considering as it is the quick method of gaining the requisite soil depth. And, as the topsoil can arrive with added nutrients, it also gets around the issue of low soil fertility.
Secondly, it may be worthwhile considering alternative methods of cultivation. For example, this could be by using the lasagne gardening technique, where after removing perennial weeds, layers of organic matter, cardboard, grass clippings etc are added to gradually deepen and improve the top layers of soil by the action of worms and decomposition.
Raised beds are another possibility, and would be neater and more suitable in formal areas of the garden. Topsoil may need to be brought in, so the cost of building the raised beds and fresh soil would need to be factored in.
Thirdly, breaking up the chalk to enable the plants to spread out their roots more easily and establish more quickly. This option will still require organic matter and / or top soil, but could work where small, young plants are being planted.
What to Grow in Chalk Gardens
There are many popular plants that naturally grow on chalky and limestone soils.
On shallow soils, especially chalk gardens in coastal areas, consider the qualities that native plants have to lead to some exciting choices for the garden.
Generally speaking, chalky soils are best suited to plants that like sharp drainage and are not too greedy in terms of nutrient requirements.
For example, Pinus mugo, the dwarf mountain pine is happy on a chalk soil and with salt air and could look delightful encircled by Lavender with Calendula adding a summer splash of contrasting colour.
Deeper chalk soils will give a wider range of plants, as the soil tends to be more moisture retentive even before the addition of organic matter.
The lists below are just to give a flavour of plants to grow in a chalk garden.
Trees
Arbutus unedo
Buxus and Yew for topiary
Flowering cherries
Gleditsia
Hawthorn
Tamarisk
Flowering Shrubs
Buddleia
Ceanothus
Deutzia
Erysimum
Hydrangea
Oleander
Osmanthus x burkwoodii
Rosa rubiginosa
Syringa
Weigela
Climbers and wall shrubs
Akebia
Clematis
Cotoneaster
Lonicera
Parthenocissus
Herbaceous Perennials
Dianthus
Doronicum
Erodium
Geranium
Hemerocallis
Hosta
Iberis sempervirens
Miscanthus sinensis
Nepeta
Rudbeckia
Thalictrum
Bulbs, Corms, Tubers
Chionodoxa
Colchicum
Crocus
Annuals
Calendula
Cornflower
Sweet alyssum
Sweet peas
The Kitchen Garden
Phacelia is often grown as a green manure on chalk soils. It is also worthwhile letting some of the green manure crop flower as phacelia is excellent at attracting pollinators.
Clover is another good green manure crop for chalk gardens that can also help reduce soil erosion over the winter due to its extensive root system.
Vegetables
Brassicas in particular as the lime in an alkaline soil reduces the problems of clubroot, so for example –
Brussels sprouts, Cabbage, Cauliflower, Pak choi
Most of the legume family (Fabaceae) prefer alkaline soils so peas and beans of all types will thrive. Growing these has the added advantage of fixing nitrogen in the soil which the next crop can take advantage of. This is particularly useful when using a crop rotation system in the vegetable garden.
Artichoke
Asparagus
Garlic, Leeks
Mushrooms
Okra
Spinach
Sweetcorn
Swiss chard
Fruit
Apple
Black Mulberry
Cherry
Kiwi
Pear
Herbs
Borage
Lavender
Marjoram
Parsley
Rosemary
Peppermint
Thyme
Yarrow
A chalk garden with a deep layer of alkaline clay soil would be a delight to garden in. The problems occur where the pH is particularly high and the soil depth is shallow.
Marie Shallcross
Plews Garden Design and Consultancy; Gardening Lessons