Native shrubs that need acidic soil: blueberries, azaleas, laurels and their kin

The heaths all need acid soil: blueberries and huckleberries, azaleas and rhododendrons, mountain laurel and the other laurels, the fetterbushes and doghobbles, and sourwood, a tree of the same family. Most grow best between pH 4.5 and 6.0. Their feeding roots are fine and shallow, and they depend on fungi in those roots for water and minerals in poor, acid ground. Lime starves them of iron, wet clay rots them and deep planting smothers them. A heath goes in acid soil that drains, set a little high under a mulch of leaves or pine needles.

Which native shrubs need acid soil

Every plant in the tables belongs to the heath family. The tables group them by soil moisture, low to tall within each. Blue Whale Trees grows all of them from seed. They leave out the heaths of 3 feet or less, from trailing arbutus to black huckleberry, which low native shrubs and groundcovers sorts by site. They also leave out more heaths of the Southeast, such as white wicky, found only in the Carolinas.

Dry to average soil

Average soil, moist but drained

Average to wet soil

Soil from dry to wet

How to tell whether your soil is acid enough

Only a soil test tells. Take slices about 6 inches deep from 8 to 10 spots across the bed, mix them into one sample and take it to your county extension office. Test a bed beside the house apart from the lawn, since lime washing out of concrete and masonry raises the pH near a foundation, a walk or a wall. Heaths are often planted in that very ground:

“Ericaceous plants are acid-loving […], yet they are frequently used as foundation plants in landscapes where the soil pH is considerably higher than what these plants typically prefer.” (Penn State Extension, Consider Soil pH Before Selecting Trees and Shrubs for Landscape Use)

The blueberries and huckleberries want the low end of the heaths’ range, about pH 4.0 to 5.5. Roseshell azalea, mountain fetterbush and farkleberry take soil closer to neutral than most.

At pH 5.3 to 6.0, sulfur spread six months before planting brings a blueberry bed down. Above 6.0 the bed needs large amounts of peat moss or milled pine bark. Soil over limestone resists the change, so choose other plants there. Yellow leaves in summer sets out what sulfur and chelated iron can and cannot fix.

Why lime, wet clay and deep planting kill them

A heath feeds through a mat of fine roots near the surface. Most azalea roots lie in the top foot of soil. When William Laycock dug up five black huckleberries whole in the New Jersey Pine Barrens for a 1967 study, their underground stems ran mostly through the leaf litter and topsoil.

Blueberry roots carry few root hairs and take up water and nutrients poorly on their own. Fungi living in the finest roots, the ericoid mycorrhizae, make up the difference. Mountain laurel depends on its fungus for water and minerals in poor, acid soil, and trailing arbutus needs a partner fungus to thrive.

Each killer works on those roots:

• Lime. On soil above its range, a heath cannot take up enough iron. Its newest leaves yellow between green veins, and the stressed plant falls more easily to pests and disease. Lime, wood ash and limed potting soil raise the pH, and hard or salty water kills the root fungi that rabbiteye blueberry depends on.
• Wet clay. Fine roots need air. In ground that stays wet they go short of oxygen, and root rot follows.
• Deep planting. Roots buried below their depth suffocate, and setting a plant too deep favors crown rot, which browns the wood of the lower stem.

How to plant a heath

Most azaleas and rhododendrons do best in part or dappled shade, out of afternoon sun, which scorches their leaves. Blueberries and sourwood flower and fruit best in sun. Then:

1. Plant while dormant, from leaf fall until the buds swell in spring.
2. Dig wide and shallow: a hole two or three times as wide as the root ball and no deeper.
3. Keep the root ball whole, since the fine roots tear. Loosen only the outer roots where they circle a pot.
4. Set it a little high, with the top of the root ball at the soil line or slightly above it. On heavy or wet ground, plant on a low mound or a raised bed 6 to 12 inches high.
5. Backfill with the soil you dug, with a little peat moss mixed in for a blueberry. On clay, work composted pine bark through as wide an area as you can.
6. Mulch 2 to 3 inches deep, kept off the stems, with pine needles, pine bark or leaves. Blueberries take 3 to 4 inches.
7. Water every day for the first two weeks, then every two to three days until week 12, then weekly through the first two summers when rain falls short. The shallow roots dry out fast, and the watering calculator gives the amounts.

Feeding: little fertilizer and no lime

Give no fertilizer at planting, since it easily damages young blueberries. After that, feed only for a reason: deerberry, dangleberry and Labrador tea need none, and overfeeding an azalea invites root rot. Blueberries grown for fruit take a light feeding once their leaves open in spring.

When a heath needs feeding, use a fertilizer for acid-loving plants or ammonium sulfate. Young blueberries take up nitrogen more efficiently as ammonium, and ammonium sulfate is the most acidifying of the common nitrogen fertilizers, as organic or synthetic fertilizer explains.

Root rot and lace bugs

Phytophthora root rot and lace bugs are among the commonest troubles of azaleas and rhododendrons. Phytophthora is a water mold of poorly drained, wet ground. A wilting plant is usually the first sign, though drought looks the same. The roots of an infected plant are soggy or blackened, and their outer layer pulls away. No spray cures it then:

“Disease prevention is important because chemical controls are ineffective once symptoms appear in the landscape.” (Clemson Cooperative Extension, Azalea & Rhododendron Diseases in South Carolina: Identification, Prevention, and Treatment)

The mold lives on in the soil, so replace a lost heath on a raised bed or on ground that drains. For a wet corner, swamp doghobble takes wet, acid ground where most shrubs rot, and root rot troubles swamp azalea less than most azaleas.

Lace bugs feed under the leaves and stipple the upper side with pale spots. Evergreen azaleas are the azalea lace bug’s likeliest hosts, but deciduous azaleas and mountain laurel are attacked too. Azaleas in full sun are the most susceptible, and a heavy infestation can kill a small shrub, so a half-shaded, airy site dodges the worst of it. Mountain fetterbush is left alone. Spots, powder and speckles on tree leaves tells lace bug damage from the spider mites’ look-alike.

Where to start

Start from the soil, then the spot:

• Test the soil through your county extension office before you buy.
• Find your soil’s table above, then the rows that take your light.
• Check a plant against your yard with the site check, which grades any heath in the tables against your zone, sun and soil moisture. It cannot see pH, which is why the test comes first.
• On limy ground, plant the trees that take limy ground instead of fighting the soil.
• Browse the heaths among the native shrubs in the shop.

Common questions

Why are my azalea’s leaves turning yellow?

On soil that is not acid enough, yellow between green veins on the newest leaves means the roots cannot take up iron. On acid soil the cause is more often wet or packed soil, root rot or too much fertilizer. Test the pH before treating anything.

Can native azaleas grow in full sun?

Most do best in dappled or part shade, since afternoon sun scorches their leaves and lace bugs do the most harm in full sun. Flame azalea takes sun where its roots stay moist, and pinxterbloom azalea wants at least half a day of sun under high shade.

Which native trees need acid soil?

Sourwood, a tree of the heath family, insists on acid soil that drains. Pin oak, black gum and river birch need soil on the acid side too, and turn yellow on alkaline ground, where pin oak can die.

Do pine needles make soil more acidic?

Only slightly: pine needles and oak leaves add a little acidity as they decay, and a layer of them changes the soil’s pH little. Use them to keep shallow roots cool and moist, and use sulfur where the pH must come down.