Mary’s Garden Diary: growing acid-loving plants in an alkaline garden

“Iwish I had an acid soil,” is a grumble commonly heard in gardens at this time of year, when the spectacular blooms of the non-invasive garden forms of rhododendrons, azaleas and camellias or the red new growths of pieris are at their best. If you long to grow these ericaceous plants but, like me, have unsuitable alkaline soil, don’t despair, as you can still include them in your garden. Other popular ericaceous plants that you might also want to grow include summer-flowering heathers (Calluna), some magnolias, Eucryphia, Enkianthus, Fothergilla, Leucothoe and blueberries.

Ericaceous plants, also known as acid-lovers or lime-haters, don’t like growing in soils that contain lime. This means they won’t grow well in soils that have a high pH. Soils are classified as being acid or alkaline based on their pH value, which is measured on a scale of 0 to 14. A neutral soil has a pH of 7. Below pH 7 is considered acidic and above is considered alkaline. The optimum range for rhododendrons is from 4.5 to 6. If you are unsure of the pH of your soil, get a soil pH testing kit to check.

If you try to grow ericaceous plants in alkaline or limey soils, their leaves will turn yellow, a condition known as lime-induced chlorosis. Such plants will not grow or flower as well and, ultimately, will die. This happens because iron and other soil nutrients become insoluble or “locked up” in the soil at high pH so the plants can’t absorb them.

If your soil is alkaline, one solution is to grow ericaceous plants in a large pot or half-barrel, with a large drainage hole, and filled with ericaceous or lime-free compost. Growing ericaceous plants in containers also allows you to position and grow them in their optimal conditions in the garden. Most ericaceous plants, except Calluna and blueberries, prefer a position in light or dappled shade, sheltered from cold winds and where the plant is not exposed to morning sun. This is important to prevent a late spring frost from causing damage. Make sure to keep pots watered, especially in dry weather, and feed regularly with a liquid fertiliser, specifically for ericaceous plants. Be aware that if you are continually using hard or limey tap water to water your plants, you will gradually raise the pH and your plant will eventually show signs of chlorosis. Watering with soft or collected rainwater prevents this problem. To increase or maintain acidity, you can also feed with sequestered iron tonic once a year in spring.

If you want to grow perhaps just one rhododendron with other shrubs in a bed of alkaline soil, this can also be achieved by planting it into a large plastic container with ericaceous compost. Dig a hole big enough to accommodate the pot in the border where the plant is required and plunge the pot into the hole so that the rim is just above soil level to prevent limey water from the surrounding soil running in.

A better, long-term solution, which works well in our garden, is to create raised beds filled with an ericaceous compost or lime-free soil. Most ericaceous plants are shallow rooting, so a bed raised to around 45cm above the existing soil is deep enough for the root system and will prevent alkaline water flowing in from the natural soil. The walls can be built of railway sleepers, half-round timber edging, brick or lime-free stone.

Potting up rooted cuttings

In July last year, I took semi-hardwood cuttings of several shrubs and old roses from around the garden including philadelphus, sambucus, abutilon, hydrangeas and more. Two or three cuttings, generally 8cm to 10cm long, were inserted into 7.5cm pots filled with seed and cuttings compost and covered over with polythene. Most were showing signs of rooting after around four to six weeks. The polythene cover was then removed, and the cuttings were overwintered in their pots in the polytunnel. I now have a lovely batch of rooted cuttings ready to move into larger pots.

These are carefully removed from their rooting containers and gently separated, taking care not to damage the new roots. Each one is potted individually into a container of nutrient-rich potting compost. When potting, I pinch out the shoot tips to encourage lots of side shoots and remove any flowers to keep the energy focused on developing a strong, bushy plant. The newly potted plants are kept watered and are grown on in the greenhouse or polytunnel until they are strong enough and ready for hardening off, either for growing on outdoors or planting out into the garden. Some slower-growing shrubs will require potting on again into larger pots to mature further before being ready for planting out.

Timely Reminder: Sow seeds of cleome

The white, spidery blooms of cleome caught my eye in a garden I visited last summer. It’s a tall-growing annual with exotic appeal and I want to include some in my borders this year for a boost of colour in late summer. The unique flowers also come in varying shades of pink.

As they are tender plants, I will need to raise them under cover in my polytunnel. Cleomes prefer good light levels and the seeds germinate most quickly if sown in warmth quite late in the spring, ideally in April or early May. Once the seedlings are about 5cm tall they can be pinched out to encourage side shoot development resulting in bushier plants. Pot up seedlings into 10cm pots to grow on. Make sure to harden off the plants, gradually acclimatising them to outdoor conditions for at least a week, before planting out in a sunny, sheltered position after the risk of frost has passed. Protect the plants from strong winds, staking if necessary.

Mary Keenan and Ross Doyle run Gash Gardens, Co Laois, open to the public. gashgardens.ie

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