Also referred to as ericaceous gardens.
If the garden is near granite outcrops in the western half of the British Isles then it is highly likely to have acid soil. This is because the combination of granite or shale bedrock on which the soil lies, plus abundant rainfall, creates the conditions that give rise to an acid soil.
Acid Soil
Soil acidity can be caused by a number of factors but rainfall and parent rock are the two which are easiest factors to use in establishing if a soil is likely to be acidic.
Soils in areas with large amounts of rainfall tend to be acidic because the water leaches basic cations (minerals) – calcium, magnesium, sodium, and potassium – out of the soil. These are minerals which are beneficial to plants and accessed through the soil. Rain itself is moderately acidic but the degree to which this will affect the soil’s pH depends on the buffering capacity of a given soil. The more organic matter within the soil the more able it is to buffer.
Peat is formed by the accumulation of incompletely decomposed plant material over a long period, in fens and bogs, ie poorly draining areas, predominantly those with cool summers and cold winters. One metre of peat depth takes approximately one thousand years to form and peat bogs may be 4 – 10 metres deep.
Gardening in an Acid Soil
An acid soil is defined as one where the pH is less than 7. The pH scale is logarithmic, meaning that pH 5 is ten times more acidic than pH 6; whilst pH 4 would be ten times more acidic than pH 5 and a hundred times more acidic than pH 6.
Disadvantages
- A peat soil has low fertility, and this can be true of other acidic soils
- Organic matter decomposition by soil organisms slows down as soils become more acidic
- Microbial activity drops off in acidic conditions which can lower nitrogen concentrations. Nitrogen is one of the three most important plant nutrients
- As acid soils are more often found in areas of high rainfall this can result in nutrients and minerals leaching out of the soil
Advantages
- Not all acid gardens have very acid soil
- A peat soil is high in organic matter
- Many plants thrive in a soil pH of 4.5 – 5.5
- Even more plants thrive in pH of 5.5 – 6.5
Raising the pH of an Acid Garden
Although it behoves us to garden as sustainably as possible and work with what is there, it is sometimes necessary to increase the pH. This is generally referred to as ‘liming’ or ‘soil sweetening’ and is the addition of calcium carbonate or a similar calciferous substance. For example: –
- Limestone, calcitic lime (calcium carbonate), offered as powder or crushed granules
- Dolomitic limestone (magnesium calcium carbonate), offered as powder or crushed granules
- Quicklime, a processed, highly unstable calcium compound
- Hydrated lime, slaked lime, fluid lime
- Marl, mudstone, basically a rock made of lime and clay
- Chalk, ie limestone
- Oyster shell, limestone is a sedimentary rock largely made from sea shells. However, using ground oyster shells which are an industrial by-product is considered a more sustainable way to add lime to acid gardens
- Wood ashes, pH level will vary between pH 8 – 9, also contains potassium
- Spent mushroom compost is useful as it adds nutrients as well as raising the pH. However some mixes include peat, which you may not wish to add
There is not room here to discuss the pros and cons of each method. Suffice to say that items such as budget, ability of staff to apply any treatment safely, and the ongoing plans for that area or garden need to be considered.
However, whichever method is chosen it will be most beneficial when applied to the root zone.
What to Grow in Acid Gardens
Whilst it may be necessary to amend soil acidity in some areas, there is a great deal to be said for working with what is there. Ericaceous gardens have a surprising range of plants which will thrive in them, many of which are coveted by those who do not have an acid garden!
Trees & Shrubs
Abies, Cedar, Fir, Pinus, most conifers
Alder
Beech
Cornus, dogwood
Elder
Holly
Acer japonicum, palmatum
Sorbus, Mountain Ash
Willow, Salix
Flowering Shrubs & Trees
Amelanchier
Azalea
Berberis
Camellia
Ceanothus
Embrothium coccineum
Fothergilla
Hamamelis
Heather, Calluna, Erica
Hydrangea
Magnolia
Pieris
Rhododendron
Styrax japonicus
Herbaceous Perennials
Begonia
Bulbinella hookeri, golden wand
Calla lily
Deschampsia, happy in pH 4.5
Gentian
Japanese anemone
Liriope muscari
Lithodora diffusa
Meconopsis, Himalayan poppy
Ornamental grasses, generally
Podophyllum
Tricyrtis formosana, toad lily
Trillium erectum
Annuals, Biennials, Bedding
In general, annuals prefer well-drained soil with a pH between 6.3 – 6.7. More importantly, bedding plants tend to need a nutritious growing media.
Bulbs, Corms, Tubers
Many will grow in less acidic soils, ie above pH 5.5,esoecially those which would naturally grow in a woodland setting.
The Kitchen Garden
Many tree fruits and bush fruits prefer a slightly acid pH. There are fewer vegetables that actively like an ericaceous environment but will tolerate a low pH if other requirements are met.
Vegetables
Potato
Radishes
Rhubarb
Sweet Potatoes
Others to try
Beans
Carrot
Onion
Squash
Sweet corn
Turnip
Salads
Cucumber
Peppers, sweet and chilli
Tomato
Herbs
These are all happy at a pH as low as 5.5 or even 5
Basil
Dill
Parsley
Rosemary
Sage
Spearmint
Thyme
Fruit
Blackberry
Blueberries
Cranberry
Elderberry
Gaultheria procumbens, Wintergreen
Gooseberry
Lingonberry
Raspberry
Others to try
Apple
Citrus
Currants
Grape
Strawberry
Rather than spending time and money to first raise and then maintain a high pH, perhaps it is better to celebrate the plants which will love an acid garden. And make use of raised beds and containers for growing plants which you may need in the kitchen garden but which need a neutral or alkaline soil.
Marie Shallcross
Plews Garden Design and Consultancy; Gardening Lessons