Plants for a Purpose – Plants for alkaline/limey soil

Soil is generally classified as alkaline or “sweet” if it has a high pH of more than 7.  The soil is classed as acid if it has a low pH of less than 7.

Soil is usually alkaline if the underlying bedrock is chalk or limestone-based. This means that the soil will often be very free-draining and lower in nutrients than more neutral pH soils.

If lumps of chalk appear when digging the soil that’s often a strong indicator that the soil is alkaline. But to be sure, buy a soil-testing pH kit from your local Garden Centre. The pH of your soil can make a big difference to the plants that will grow well in it.

A main reason why plants that grow well in acid or neutral soils don’t do well in alkaline ones is because their roots can’t absorb certain nutrients important to their growth and health. Such as iron. It’s important to choose the right plants for this very specific soil type of alkaline soil.

Very alkaline or chalky soils often dry out quickly.  Check the soil depth too, as these types of soil can be shallower with more chalk or rocks than soil!

If you’d like to plant a tree, then look for ones that aren’t fussy about soil depth, water availability or nutrients.

Crab apple, hawthorn and spindle trees will all grow well in these conditions. And they won’t dominate the garden or cause excess shade as they mature.

The Judas tree (Cercis siliquastrum) will also grow extremely well in these conditions. They have the bonus of producing beautiful purple-pink flowers in spring.  You’ll see lots of these trees growing wild in the French countryside. The sight of them all in bloom is stunning.

Cercis canadensis ‘Lavender Twist’ (Redbud Lavender Twist)

How about a weeping form of the redbud?  It’s ideal for smaller gardens and, not growing so tall, there won’t be so many roots needed to support the tree’s stem which should help it establish well in a chalky soil garden.  Come spring the bare branches become laden with stunning lilac-pink flowers and, as they fade, their place is taken by heart-shaped vibrant green, leaves which in turn change to bright, buttery shades of yellow before falling in the autumn months.  The other good news is that it doesn’t require pruning!

If you wanted to grow a fruit tree then apples, pears and cherry trees all grow well on alkaline soil.

Fortunately, many shrubs thrive in alkaline soil.  Berberis, cotinus, lavender and potentilla are all very happy growing in these soil conditions.  If year-round presence is required, there are several evergreen plants of varying heights too. Such as euonymus and arbutus, the strawberry tree, so called because of the colour and shape of its fruit. These fruits appear during the autumn months.  If you’re looking for an evergreen shrub that also flowers, then Californian lilac (Ceanothus) is a great choice.

Ceanothus ‘Autumnal Blue’ (Californian lilac)

This handsome, evergreen shrub has fluffy clusters of sky-blue flowers and small, glossy, bright green leaves. It has a strong, upright habit and is ideal for training on a sunny wall. Or even for the back of a south or west-facing shrub border. It is one of the hardier varieties of Californian lilac. However, it will need protection from strong wind so make sure to plant in a sheltered position in the garden.

Achillea, campanula, phlox and salvias will all grow well in alkaline soil.

Achillea are long-lasting, easy to grow and available in a wide range of both bright and pastel colours. This makes them a great choice for this soil type.

Achillea millefolium ‘Lilac Beauty’ (Yarrow)

This variety delights with a profusion of lilac flowers all summer long, fading to a delightful lavender pink with time.  Looks great growing next to dark purple flowered salvias and attracts many types of pollinating insects.

Geranium phaeum (Dusky cranesbill)

For shadier areas, look for shade-loving varieties of campanula or geranium. A lovely, easy-to-grow plant that delights with handsome, green leaves covered in deep purple markings. And, in late spring/early summer, flowers that are deep maroon colour with white centres.  The plant will soon bulk up and can be divided in spring to increase the number of plants you have.

Many of the Mediterranean woody-stemmed herbs, like lavender, rosemary, and thyme, grow well in alkaline soil if given full sun. The free-draining nature of the soil suits them perfectly!  Avoid French or Spanish lavender if your garden is in a colder or more exposed area of the country.

Lavandula anguvstifolia ‘Munstead’ (Lavender)

This wonderfully compact grey-green leaved cultivar produces dense clumps of deep purple, scented flower spikes great for cutting and drying.  Ideal for path edges, where brushing against the plants releases a delightfully refreshing fragrance and also for the smaller garden.

Origanum ‘Rosenkuppel’ (Oregano)

Many woody stem herbs such as oregano, marjoram, and thyme flourish too. And they are easy to grow, attracting many pollinating insects when in flower. This superb plant produces gorgeous, deep purple flowers during the summer and has handsome, small, rounded dark leaves. Pollinating insects adore the flowers and flock to them, particularly on sunny days.

Our native honeysuckle will grow well. There are lots of cultivars from it such as ‘Serotina’ and ‘Graham Thomas’ to choose from.  A more self-contained evergreen climber is star jasmine, which will also produce scented flowers during the summer.

Trachelospermum jasminoides (Star Jasmine)

An easy-to-grow, evergreen climber with dark green leaves which often turn bronze during the winter months.  Once established it will produce highly scented, pure white flowers, in mid to late summer. In colder areas, grow against a warm, sheltered wall.

Several types of geraniums will quickly cover large areas of ground. Or, for a shady area, why not try the sweetly scented lily of the valley?

Convallaria majalis (Lily of the Valley)

These plants will tolerate a broad range of soils and in late spring produce sweetly perfumed, white, nodding, bell-shaped flowers dangling daintily from thin stems amidst upright, broad leaves.