Azaleas are a common plant in the South Carolina landscape. There are numerous azalea species, hybrids, and cultivars that are either native to this area or to the Middle East. Azaleas are in the genus Rhododendron and bloom times range from the early spring to midsummer.
Azaleas prefer to be planted in partially shaded sites. Azaleas do better planted beneath pine trees than hardwoods because they are not competing for nutrients and water with the shallower roots of a hardwood tree. Some varieties of azaleas will tolerate sun better than others; however, all azaleas prefer an area that is not exposed to long periods of hot full sun and drying winds. Flowers last longer when plants are partially shaded. Azaleas exposed to full sun all day are more susceptible to lace bugs. Early morning sun exposure after a hard freeze may cause cold injury. Azaleas planted in heavy shade can result in poor flowering and weak growth habit.
Azaleas grow best in acid (4.5 to 6.0 pH), well-drained, organic soils. Before planting, have the soils tested and adjust the pH according to soil test results. Azaleas are shallow-rooted plants that are easily damaged by excessive soil moisture. Azaleas planted in poorly drained sited do not receive the oxygen required for healthy growth and often develop root rot diseases. When planting in poorly drained areas, plant the root ball higher than ground level, making sure the root flair is not below the ground level. The root flare or root collar is the base of the shrub or tree when the trunk transitions from trunk and bark tissues into the root system tissues. Roots should be growing outwards from the base of the azalea at the same level as the surrounding soil or above the soil in poorly drained areas.
Adding 2- to 3-inch layer of organic mulch such as composted pine bark, pine straw, and leaves to as large an area as possible will also be helpful. Mulch conserves soils moisture, maintains soil temperature, and helps discourage weeds. As organic mulch decomposes it enriches the soil with organic matter. Keep the mulch a couple of inches away from the main stem to keep the bark dry and to deter voles.
Since azaleas are a shallow-rooted plant, they will require irrigation during dry periods. Azaleas planted in warm weather in sandy soils may require watering of the root mass twice a week during the first year. To determine when to water, pull back a small area of mulch near the base of the plant and check the moisture level of the root ball and surrounding soil. If the top few inches of soil feels dry, wet the soil deeply to at least 6 to 8 inches deep.
Azaleas have low nutritional requirements compared to other shrubs. Soils amended with organic matter prior to planting followed by a mulch of organic material will usually provide sufficient nutrients for adequate growth.
There are two reasons to fertilize: to increase the growth rate or to correct a nutrient deficiency. A nutrient deficiency can have symptoms such as stunted growth, smaller than normal leaves, light green to yellowish leaf color, or early leaf drop. These same symptoms can be caused by other problems such as compacted soils, stress from insects, disease organisms and weeds, and excessively wet or dry soils. Fertilization will not correct those problems, so be certain that you know the cause of the symptoms and treat them appropriately.
Having your soils tested is one way to determine if applying fertilizer will benefit your azaleas. In the absence of a soils test fertilize azaleas lightly in the spring and early summer with a complete, extended-release, acid-forming, azalea fertilizer. Look for a fertilizer with nutrients in a ratio close to 2-1-1, such as a 10-5-4.
Apply fertilizer to the azalea’s root zone area which can extend beyond the drip line or outer-most branches. A shrub’s roots can extend three times the distance from its center to the outer most branches. Evenly broadcast the fertilizer with a handheld spreader or rotary or cyclone spreader over the root zone area. Brush off any fertilizer off the branches and water afterwards to make the nutrients available to the roots. Azaleas can be fertilized any time from late spring, after the new growth emerges, up to early summer. Avoid fertilizing plants stressed by drought during the summer months. Without water, plants are unable to absorb nutrients, so it is best not to fertilize if water is unavailable.
Azaleas may be pruned by thinning or heading. Thinning refers to the removal of branches back to the main truck or another branch. This method is used to remove leggy branches that extend beyond the canopy, remove damage or diseased wood, or reduce the size of the plant. Thinning allows light to enter the shrub and encourage growth on the interior branches. You can thin at any time of the year without causing significant impact of flowering, growth, or cold hardiness of the plant. However, to reduce the impact on flowers the following year, prune after flowering in the spring.
Heading refers to the cutting back of a branch, not necessarily to a side branch. This method is used to reduce the size of a plant, create a hedge, or to renew old overgrown plants. Renew overgrown plants by cutting them back to within 6 to 12 inches of ground level. This practice will result in lavish new growth by midsummer.
The best time to renew azaleas is before spring growth beings. This allows plenty of time for next year’s flower buds to form in midsummer, and for new growth to mature and harden of for winter. Renewal pruning before spring growth means that flowers are sacrificed for that year. After renewal pruning, prune the tips of new shoots when they are 6 to 12 inches long to encourage branching and a full canopy. Thin out new shoots emerging from the old stem. Remember to keep the soil moist during the period after severe pruning.
For more information on azaleas, please visit the Home and Garden Information Center website at hgic.clemson.edu. Tune in on Tuesday nights to watch “Making it Grow” at 7pm on SCETV or mig.org. Email Outen at callenb@clemson.edu.
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