Woody Plants for Acidic SoilsBy Todd Boland (Todd_Boland)
January 10, 2009
Most acid-loving shrubs prefer dappled shade, but can tolerate full sun if the soil is reasonably moist. Many shrubby selections are evergreen so they will perform best in a sheltered location. These evergreens are very susceptible to winter burn due to the cold, dry, desiccating winds of January and February. In early spring, strong winds (even if not too cold) together with strong sun can result in leaf scorch. Therefore, western, north-western or northern exposures are poor locations for these shrubs unless they are surrounded by taller shrubs or trees. An eastern to southern exposure is ideal. Deciduous shrub selections can better tolerate exposed locations, but even they benefit from at least some shelter in colder regions.
Generally, acid-loving shrubs prefer soil which is rich in organic matter. If your soil is naturally acidic, you will not need to add an acidifying agent like aluminum sulfate. The soil should retain moisture, yet be well-drained. The following soil mix can be recommended:
2 parts peat
1 part loam
1 part leaf mould
1 part coarse sand
Old compost or manure is also beneficial and may be used as a replacement for leaf mould.
If the plant is root-bound, gently pry apart the outer roots to help loosen them. If very pot-bound, then use a sharp knife and make three or four, 2 to 5 cm longitudinal wedges from the root ball. This may seem drastic but the severed roots will stimulate the production of new roots. If planted in the root-bound condition, the plants will be very slow to become established.
To plant, add about 15 cm of prepared soil into the planting hole. Then position the plant in the hole and backfill with prepared soil. The shrub should be at the same depth as it was in its container.
It is beneficial to place a 3-5 cm layer of leaf mould or old compost around the base of a newly planted or established shrub. This mulch will help to maintain soil moisture, keep the ground cool and keep weeds to a minimum.
Care and Maintenance:
During dry spells it will be necessary to water your plants; an inch of water once a week is the recommended rate. Avoid light sprinklings of water as this only encourages shallow roots.
Fertilizers may not be needed if your mulch is from compost or old manure. If the shrub does not seem to be thriving, the application of a general fertilizer such as 5-10-5, at manufacturer's recommended level, can be added in spring. You can also purchase fertilizers especially formulated for acid-loving shrubs, but I must admit, I find 5-10-5 to be as good and often cheaper than the ‘acid' fertilizers. Place the fertilizer around the plants at their dripline and scratch into the soil surface. This should be done just as the buds swell in spring.
Pruning may never be required unless you are trying to limit the size of the plant involved. If the plant is grown primarily for its flowers, then the general rule is to prune spring-flowering shrubs as soon as their flowers fade and prune mid-summer to fall bloomers in early spring. Rhododendrons and azaleas produce flower buds in the summer prior to flowering. Thus, any pruning in summer or fall may result in the removal of next year's flowers. The only yearly pruning which needs to be practised on these is the prompt removal of faded flower heads. Cut these faded trusses just above the uppermost leaf.
In northern areas, winter protection is highly recommended. There are several ways to achieve this protection. Smaller shrubs can be protected by placing evergreen boughs around the plants. A teepee-effect with lattice works well too.
Larger broad-leaved evergreens can have a boxed framework built around them. This frame is then covered in burlap. DO NOT USE PLASTIC. The idea is not to warm the shrubs, but to protect them from excessive wind and sun. Do not tightly wrap broad-leaved evergreen shrubs in burlap, as is often done with other evergreens, such as yews or cedars. Deciduous shrubs may simply need to be tired together in winter to prevent snow from splaying the plants.
Alternatively, you can select the toughest plants for your region and avoid all this winter fuss but if you are like me, you like to push the limits, thus winter protection becomes the norm.
The following groups of plants are among the best performers under acidic soil conditions:
Heath/Heather - Erica and Calluna
Examples of Azalea, Rhododendron, Heather and Heath
Magnolia species and cultivars
Japanese Pieris - Pieris species and cultivars
Mountain Laurel - Kalmia latifolia
Examples of Magnolia, Japanese Andromeda and Mountain Laurel
Japanese Maple - Acer palmatum
Amur Maple - Acer tatarica
Cotoneaster species and selections
Examples of Japanese Maple, Amur Maple and Cotoneaster
Bearberry - Arctostaphyllos uva-ursi
Bog Rosemary - Andromeda glaucophylla
Examples of Bearberry, Conifers and Bog Rosemary
Holly - Ilex species and cultivars
Crabapple - Malus cultivars
Mountain-ash - Sorbus species and selections
Examples of Holly, Crabapple and Mountain-ash
Chokeberry - Aronia species and cultivars
Peashrub - Caragana species and cultivars
Dogwood - Cornus species and cultivars
Examples of Chokeberry, Seashrub and Dogwood
Wintergreen - Gaultheria species
Witch-hazel - Hamamelis species and cultivars
Hydrangea species and cultivars
Examples of Wintergreen, Witch-hazel and Hydrangea
Shrubby Cinquefoil - Potentilla fruticosa
Viburnum species and cultivars
Drooping Leucothoe - Leucothoe fontanesiana
Examples of Shrubby Cinquefoil, Viburnum and Leucothoe
Serviceberry - Amelanchier species
Pin Oak - Quercus palustris
Examples of Serviceberry and Pin Oak