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  3. Rhododendron (Azalea)

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Plant:

Rhododendron (Azalea)
Image of Rhododendron (Azalea)
Flowering
Late winter to early summer. A few bloom in mid-late summer.
Adaptation
Throughout the United States, except in desert areas. Azaleas are more tolerant of warm, dry climates than rhododendrons are.
Pruning
Prune just after flowering. Azaleas: For bushy plants, pinch off tips. Rhododendrons: Remove dead flower trusses; be careful not to break new buds.
Soil
Rich, well-drained soil that''s high in organic matter. When planting, keep 2 inches of mulch around the base of the plant.
Water
Water when the soil is moist but not wet under the mulch.
Fertilizer
Fertilize with plant food for acid-loving plants.
Light
Partial shade (full sun in cool summer areas). Plants bloom best in partial sun.
Related problems

Iron Deficiency - Rhododendron (Azalea)

Image of Iron Deficiency - Rhododendron (Azalea)
Iron Deficiency

Lace Bugs -

Image of Lace Bugs
Lace Bugs

Rhododendron Aphid -

Image of Rhododendron Aphid
Rhododendron Aphid

Root Weevil Adults -

Image of Root Weevil Adults
Root Weevil Adults

Wilt and Root Rot -

Image of Wilt and Root Rot
Wilt and Root Rot

Windburn and Winter Injury -

Image of Windburn and Winter Injury
Windburn and Winter Injury
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Iron Deficiency - Rhododendron (Azalea)

Image of Iron Deficiency - Rhododendron (Azalea)
Iron Deficiency
Description
The leaves are pale green to yellow. The newest leaves may be completely yellow, with only the veins and the tissue right next to the veins remaining green. With progressively older leaves, only the leaf edges may be yellowing. The plant may be stunted.
Analysis

This is a common problem in acid-loving plants such as azalea and rhododendron. These plants prefer soil with a pH between 5.0 and 6.0. The soil is seldom truly deficient in iron, but iron is often found in an insoluble form that is not available to the plant, especially in soil with a pH above 7.0. A high soil pH can result from overliming or if lime has leached from cement or brick. Regions where soil is derived from limestone or where rainfall is low also have high-pH soils. Plants use iron in the formation of chlorophyll in the leaves. When iron is lacking, new leaves are yellow.

Solution

Fertilize with a plant food formulated for acid-loving plants. If your soil drainage is poor, add enough peat moss at a 50:50 ratio to native soil when planting. Never lime the soil around this plant.

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Lace Bugs -

Image of Lace Bugs
Lace Bugs
Description
The upper sides of leaves are mottled or speckled yellow and green. Hard, black, shiny droplets are on the undersides of damaged leaves. Small, spiny, wingless insects or brownish insects with clear lacy wings may be seen around the droplets. Plant growth is usually stunted. Damage occurs in spring and summer.
Analysis

Stephanitis species Populations of lace bugs are highest when rhododendrons and azaleas are grown in sunny rather than shady locations. The wingless, immature insects and the lacy-winged adults suck sap from the undersides of leaves. As they feed, droplets of black excrement accumulate around them. Damage is unsightly, and food production by the leaf is reduced, making the plant less vigorous.

Solution

Apply an insecticide labeled for this pest, following label directions.

Related Products
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Ortho® MAX® Malathion Insect Spray Concentrate

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Rhododendron Aphid -

Image of Rhododendron Aphid
Rhododendron Aphid
Description
Tiny, green or pink, soft-bodied insects cluster on the leaves and developing buds. Flower buds may be deformed, and the leaves often have curled edges. A shiny or sticky substance usually coats the leaves. A black sooty mold may grow on the sticky substance. Ants may be present.
Analysis

Macrosiphum rhododendri Rhododendron aphids do little damage in small numbers, and plants can tolerate fairly large numbers without much effect. Aphids are extremely prolific, however, and populations can rapidly build up to damaging numbers during the growing season. Damage occurs when the aphid sucks the juices from the rhododendron leaves and buds. The aphid is unable to digest fully all the sugar in the plant sap and excretes the excess in a fluid called honeydew. The honeydew often drops onto the leaves below. A sooty mold fungus may develop on the honeydew, causing the rhododendron leaves to appear black and dirty. Ants feed on the sticky substance and are often present where there is an aphid infestation.

Solution

When you first notice aphids, apply an insecticide labeled for this pest, following label directions.

Close

Root Weevil Adults -

Image of Root Weevil Adults
Root Weevil Adults
Description
The leaf margins are scalloped or notched. The leaves may be green and healthy, or yellowing and curled. To check for this insect, inspect the foliage after dark, using a flashlight. Black or grayish insects, 1/5 to 2/5 inches long, with elephantlike snouts, are feeding on the leaf edges. These insects are present from May or June to as late as September.
Analysis

Root weevils are common pests of many garden plants. The adult weevils feed at night, notching leaf margins. This feeding detracts from the appearance of the plant but rarely causes serious injury. Severe damage and death of the plant may result from larvae feeding on roots if the weevils are left uncontrolled. The grubs feed unseen on the roots in the spring and from mid- or late summer into fall.

Solution

To control adults, apply an insecticide labeled for this pest, following label directions.

Close

Wilt and Root Rot -

Image of Wilt and Root Rot
Wilt and Root Rot
Description
The young leaves are yellowish and wilting. Eventually the whole plant wilts and dies, even though the soil is moist. Dead leaves remain attached to the plant and are rolled along the midrib. Heavy, poorly drained soil favors disease development. The symptoms may develop over a few weeks or may take many months.
Analysis

This plant disease is caused by several different soil-inhabiting fungi, also known as water molds. These fungi (Phytophthora and Pythium species) attack a wide variety of ornamental plants, where they destroy roots and work their way up the stem. If they girdle the stem, the plant wilts and dies. Very wet conditions favor the fungi, which are most common in heavy, poorly drained soils. Azaleas and rhododendrons need constant moisture, but they require good drainage.

Solution

Control of root rot is difficult. Improve soil drainage before replanting azaleas or rhododendrons in the same location. If drainage cannot be improved, plant in beds raised a foot or more above grade. Or, plant shrubs that are resistant to wilt and root rot.

Close

Windburn and Winter Injury -

Image of Windburn and Winter Injury
Windburn and Winter Injury
Description
Young and exposed leaves are brown and dry, especially around the leaf margins and near the tips. The shrub is planted in a windy location or is growing in a climate where cold, dry, windy days are common. The soil may be frozen or dry.
Analysis

Windburn and winter injury on rhododendron and azalea leaves are common on plants growing in windswept locations. In cold climates, where temperatures commonly fall below freezing, strong winds cause leaves to lose their moisture more rapidly than it can be replaced by the root system. The leaf edges dry out and die. This is most pronounced when water is unavailable because the soil is frozen. Leaf burning also occurs on exceptionally windy, dry days in summer.

Solution

Once leaf edges have been damaged, the injury remains. Where practical, pick off damaged leaves. To prevent winter injury and windburn, plant shrubs in locations protected from wind, or provide windbreaks. Fertilize plants rwith plant food for acid-loving plants. Water in late fall or winter to ensure adequate soil moisture. Mulch plants after they are dormant to reduce the depth of frost penetration into the soil.

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