Azaleas & Rhododendrons

Posted by Sunset Garden Experts on

azaleas & rhododendrons


Rhododendrons and azaleas, both from the genus Rhododendron, have long been mainstays of late spring because of their spectacular clusters of showy blooms. The flowers are tubular-, funnel-, or bell-shaped—and often fragrant. The leaves of the smaller azalea are usually pointed and narrow; the leaves of the rhododendron are generally large and leathery.

These shrubs prefer climates with adequate rainfall and moist summers. The two main azalea groups, evergreen and deciduous (varieties that drop their leaves in the fall) can be found in nearly every part of North America, from the frosty Canadian plains to tropical Florida. The rhododendron types are fussier, preferring environments where it is neither too hot nor too cold (Zones 5 to 8). They need a certain amount of chilling to develop strong flower buds.

Botanically speaking, all azaleas are rhododendrons because they are in the genus Rhododendron but not all rhododendrons are azaleas. Though they look similar, there is one easy way to tell them apart: Azalea flowers have only 5 stamens while rhododendrons flowers can have 10 or more in each blossom.


Known as “The Royalty of the Garden,” azaleas have long been adored for their brightly colored flowers and outstanding form and foliage. Here are a few tips for growing azaleas in your garden! Deciduous varieties are considered the hardiest, many growing as far north as Zone 4. Some, such as the bright-pink roseshell azalea (Rhododendron prinophyllum), are even hardy in Zone 3. With few exceptions, most evergreen azaleas are only reliable in Zone 6. Here at Sunset Nursery we carry only the deciduous varieties as they are the only ones hardy for our climate which is Zone 4.


  • The best time to plant azaleas is in late spring or early fall. Deciduous varieties flower more profusely in full sun.
  • Provide well-drained, humus-rich soil that is slightly acidic (pH 4.5–6).
  • Peat moss based soils such as 3 way mix are good for Rhododendrons due to peat moss's natural acidity.
  • Mulch well. Shallow-rooted, azaleas tend to dry out quickly if not mulched. A mulch of partially broken down oak leaves, or pine needles, will keep soil acidic and moist.
  • Fertilizer isn’t needed. The decaying mulch will provide all of the nutrients that azaleas need.
  • Seldom bothered by insects and diseases, azaleas require little care once established, except for watering during dry times.



In late spring, the rhododendrons are blooming! Here’s more about this stunning flowering shrub—which are easy to grow as long as you have the right conditions. Rhododendrons belong to the Erica family along with plants like heathers and heathers, and cranberries and blueberries. All Ericas have very fine, hair-like roots that form densely packed, shallow rootballs that grow as wide as the crown of the plant. In nature, they grow in acidic leaf mold and moss under deciduous trees. Since they are not tap-rooted they do not compete with the trees for nutrients, making them ideal understory plants.


  • Plant in spring or early fall.
  • Space plants 2 to 6 feet apart, depending on their estimated mature size. Dig a hole as deep as the root ball and 2 times as wide.
  • Set new plants so that their top roots are at soil level or slightly below. If you plant them any deeper, the roots may rot.
  • Fill the hole half full with soil, then water it well to settle the soil before filling with remainder of soil.
  • Peat moss based soils such as 3 way mix are good for Rhododendrons due to peat moss's natural acidity.


  • Mulch plants every spring with 2 to 5 inches of pine bark chips or pine needles to protect shallow roots and retain soil moisture. A lack of water reduces flower-bud formation.
    • Tip: A common mistake is to create a so-called “mulch volcano,” where mulch is piled heavily around the trunk of the shrub. In fact, this can keep the trunk too wet and encourage rot. Always leave a few inches around the trunk free of mulch.
  • Fertilize azaleas and rhododendrons sparingly and only when flower buds swell in the early spring. Heavy applications of fertilizer will burn the plants.
  • Water plants during the summer if rainfall is less than 1" per week.
  • After flowering, deadhead if you have the time, to promote vegetative growth rather than seed production. Remove dead flowers from rhododendrons carefully; next year’s buds are just under the old heads and will start to develop shortly after flowering.
  • In regions with severe winters, wrap evergreen rhododendrons with burlap in the fall and apply extra mulch around the base of the shrub.
  • Azaleas and rhododendrons may be transplanted at any time during the growing season, but they transplant most successfully during fall or early spring, when they are dormant and temperatures are cool.


  • In general, do not prune spring-flowering shrubs such as azaleas and rhododendrons. If you need to reduce height, prune after flowering in the spring.
  • Otherwise, just remove dead, damaged, or diseased branches at any time of year.
  • On young and old plants, simply snap off spent flower stalks by bending them over until they break away from their stems. Be careful not to damage growth buds at the base of each flower stalk.
While adding acidic soil right from the beginning is good is will typically have a hard time staying acidic unless you add something ever now and again. Rhododendrons and Azaleas love acidic soil because when the soil is acidic the nutrients which they enjoy most are more readily available to them. For this reason one of if not the most common reason for these plants to go into decline is if the soil isn't acidic enough. Our water that we use to water them (unless you use rain water) is typically very alkaline (the opposite of acidic) and over time will make the soil less acidic the more you water. 
To keep the soil acidic you can add things like pine bark needles or shredded oak leaves around the base of the plant. For an instant boost of acidity you can add aluminum sulphate around the base of the plants. Just make sure you follow the directions on the back or ask one of our experts if you are unsure as it can damage your plants if you add too much.

Share this post

← Older Post Newer Post →