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Woody Plants for Acidic Soils

By Todd Boland (Todd_BolandJanuary 10, 2009
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Do you have naturally acidic soil? Have trouble find the right woody plant for such soils? Don't worry about amending your soil - work with nature! There are many woody plants that prefer or tolerate highly acidic soil. This article will explain how to care for such plants and what choices exist.

Gardening pictureIt is a well-known fact that some plants have specific soil pH preferences; some perform best under alkaline soil conditions, other acidic yet others could care less! Gardeners in certain regions, like myself, have to deal specifically with highly acidic soils, pH 4.0 to 5.0. Many trees and shrubs are not happy under such conditions as acidic soils can ‘lock up' certain needed micronutrients. As gardeners, we can add lime to such soils to increase the pH to more acceptable levels but why not work with what we are given. As it happens, there are a number of woody plants that are naturally adapted to highly acidic soils. One group of plants are the Ericaceous shrubs such as rhododendrons, mountain laurels, heaths and heathers. There are also a number of other woody plants that can be combined with Ericaceous shrubs to provide year-round interest.

Site Conditions:

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 shelterImage in colder regions.

Soil Requirements:

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.

Planting:

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:Image

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.Image

Winter Protection

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.

Selections:

The following groups of plants are among the best performers under acidic soil conditions:

Rhododendron/Azalea

Heath/Heather - Erica and Calluna

ImageImageImageImage

Examples of Azalea, Rhododendron, Heather and Heath

Magnolia species and cultivars

Japanese Pieris - Pieris species and cultivars

Mountain Laurel - Kalmia latifolia

ImageImageImage

Examples of Magnolia, Japanese Andromeda and Mountain Laurel

Japanese Maple - Acer palmatum

Amur Maple - Acer tatarica

Cotoneaster species and selections

ImageImageImage

Examples of Japanese Maple, Amur Maple and Cotoneaster

Bearberry - Arctostaphyllos uva-ursi

Most Conifers

Bog Rosemary - Andromeda glaucophylla

ImageImageImage

Examples of Bearberry, Conifers and Bog Rosemary

Holly - Ilex species and cultivars

Crabapple - Malus cultivars

Mountain-ash - Sorbus species and selections

ImageImageImage

Examples of Holly, Crabapple and Mountain-ash

Chokeberry - Aronia species and cultivars

Peashrub - Caragana species and cultivars

Dogwood - Cornus species and cultivars

ImageImageImage

Examples of Chokeberry, Seashrub and Dogwood

Wintergreen - Gaultheria species

Witch-hazel - Hamamelis species and cultivars

Hydrangea species and cultivars

ImageImageImage

Examples of Wintergreen, Witch-hazel and Hydrangea

Shrubby Cinquefoil - Potentilla fruticosa

Viburnum species and cultivars

Drooping Leucothoe - Leucothoe fontanesiana

ImageImageImage

Examples of Shrubby Cinquefoil, Viburnum and Leucothoe

Serviceberry - Amelanchier species

Pin Oak - Quercus palustris

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Examples of Serviceberry and Pin Oak


  About Todd Boland  
Todd BolandI reside in St. John's, Newfoundland, Canada. I work as a research horticulturist at the Memorial University of Newfoundland Botanical Garden. I am one of the founding members of the Newfoundland Wildflower Society and the current chair of the Newfoundland Rock Garden Society. My garden is quite small but I pack it tight! Outdoors I grow mostly alpines, bulbs and ericaceous shrubs. Indoors, my passion is orchids. When not in the garden, I'm out bird watching, a hobby that has gotten me to some lovely parts of the world.

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Discussion about this article:
SubjectTopic StarterRepliesViewsLast Post
Acid Soil ! erichle 1 13 Jan 16, 2009 2:06 AM
Great article, beautiful photos lorettamar 1 14 Jan 12, 2009 6:53 PM
Todd Boland Avishai 0 9 Jan 12, 2009 6:07 PM
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