Azaleas are among the most popular spring-flowering shrubs – and deservedly so. Their brilliant flowers can’t be beat for early season color. Caring for azaleas isn’t difficult, but one item many gardeners struggle with is azalea fertilization. Do you know when to fertilize azaleas, what fertilizer to use, and how to apply? If not, this article provides all the know-how you need to get the job done right.
Azalea vs Rhododendron
First, you might be unsure as to whether you’re growing an azalea or a rhododendron. Let me offer some quick tips for telling the difference.
Azaleas are woody shrubs that produce a display of beautiful blooms in the spring. Depending on the species, they are native to either the United States or Asia, and there are many cultivars and varieties. Some azaleas are evergreen and hold onto their leaves year-round, while others are deciduous. Azaleas belong in the plant genus Rhododendron, but differ from the plants we commonly call rhododendrons (a.k.a. rhodies) in a few important ways.
- Azalea flowers are funnel-shaped while rhodie blooms are bell-shaped.
- Azalea leaves are small and oval shaped and cluster at branch tips while rhodies leaves are elongated, thick, leathery, and glossy
- Azaleas can be evergreen or deciduous while rhodies are always evergreen
- Azaleas have 5 stamens per flower while rhodies have 10
- Azaleas bloom in early spring in the Northern Hemisphere (April to May in my garden) while rhodies bloom later in the season (May or June)
- Azaleas produce blooms at the end of each stem as well as along the sides of the branches while rhodies produce blooms in large clusters at the ends of the branches only
The good news is that regardless of which of these shrubs you grow, the fertilization program is pretty much the same. So, yes, this article is about when to fertilize azaleas and how to do it right, but it is also appropriate info for the feeding of rhododendrons, too.
The best azalea growing conditions
Before diving into fertilization tips for azaleas, it is important to understand the environment in which azaleas evolved and the growing conditions that are best for them. Because they evolved in woodlands, beneath the dappled shade of deciduous trees, azaleas prefer rich, well-drained, acidic soils, just like the camellia, blueberry, holly, and several other evergreen shrubs.
Azaleas are at home in woodland gardens, where they make a wonderful understory plant that thrives in full to semi-shade. They often suffer when planted in full sun and are prone to infestations of lace bugs, scale, and other pests when grown in exposed, bright conditions.
When in bloom, azaleas offer a show-stopping diversity of colors, from pinks and purples to red, white, orange, and yellow. Throughout the rest of the growing season when they are not in bloom, their green leaves add texture and color to the garden. In the winter, the frost looks lovely clinging to their foliage.
Why is it important to fertilize azaleas?
Because azaleas evolved in acidic soils, they prefer a soil pH range of 4.5 to 6.0. In order for ample flower buds to develop and leaves to stay healthy and green, fertilizer is sometimes necessary, primarily to maintain acidic soil pH measurements but also to provide the shrubs with nutrients.
That being said, azaleas have fairly low nutritional requirements when compared to some other shrubs. However, without the proper soil pH, azaleas have restricted access to the iron, phosphorous, and other nutrients in the soil that are responsible for their deep green leaves and beautiful blooms (see “Signs of under fertilization” section below).
How to test the soil beneath azaleas
For azaleas, a soil test every 3 to 4 years is essential, if for nothing other than to monitor the soil’s pH and determine when to fertilize azaleas in your garden.
- Do-it-yourself soil test kits that you send off to an independent laboratory are a good and inexpensive option.
- Soil test results can also be obtained by purchasing a soil test kit from your local Agricultural Extension Service through your state’s land-grant university if you live here in the U.S.
- Probe-based soil pH testers that you insert into the soil around the base of the plants are another option for measuring pH only, though they are much less reliable than laboratory tests. You get what you pay for with these tools (in other words, cheap probes are not as accurate as more expensive models). Take 5-10 measurements in the area and then average them out for a more accurate reading. These probes do not test nutrient levels.
In addition to measuring the pH, the first two tests share existing levels of macro nutrients such as phosphorous, potassium, calcium, and magnesium, as well as micro nutrients/trace elements, including iron.
Soil tests can be taken at any time. However, I prefer late summer since the labs are not so busy and I can get the results back in time to develop a fertilization plan.
Signs of under fertilization
Nutritional deficiencies are visible in a few different ways. One sign that it’s time to think about when to fertilize azaleas is chlorotic leaves that have green veins but are yellowing in between them. Chlorosis will primarily be evident on new leaves if soil pH is an issue. If it’s a nitrogen deficiency, the yellowing will occur on the older leaves toward the inside of the shrub.
Phosphorus deficiency symptoms include leaves that turn very dark green to black with red leaf undersides, especially down the center vein. Phosphorous deficiency symptoms are often the result of incorrect soil pH, rather than an actual deficiency of phosphorous in the soil. (Are you starting to see why proper soil pH is so important for azaleas?)
Other signs of under fertilization on azaleas include a decreased growth rate, smaller leaves, early leaf drop, and/or reduced blooming. But these signs can also be a symptom of soil that is excessively wet or compacted, or even an insect or disease issue. Be sure to investigate to make sure fertilization will fix the problem. A soil test is an easy way to ensure you’re getting the answers you need. Healthy foliage is deep green on both the top and underside of the leaves.
The best fertilizers to use on azalea bushes
There are a few different types of fertilizers for azaleas.
- Acid-specific organic granular fertilizers are the best choice if your soil pH needs to be corrected and your soil test shows a deficiency in certain nutrients. They are typically not as nutrient rich than all-purpose fertilizers and have a modest amount of nitrogen, which means they are less likely to burn the roots. Products in this category include Holly-Tone and Jobe’s Organics Azalea Fertilizer Spikes.
- Soil acidifiers based on sulfur are the best choice if your soil has adequate nutrients, but the pH needs to be more acidic. Products in this category include Jobe’s Soil Acidifier, Espoma Soil Acidifier, and Ferti-lome Liquid Fertilizer and Soil Acidifier Plus Iron.
- If you have the correct pH, but are lacking nutrients, opt for an all purpose fertilizer formulated for evergreens. Products in this category include Dr. Earth Acid-Lovers and Azalea-Tone.
Synthetic slow release fertilizers and inorganic fertilizers are other options, but I don’t personally use them because I prefer naturally derived products, whenever possible.
Follow the package label for information on how much fertilizer to use with each application. Routine applications aren’t necessary. Instead, look for the signs of under fertilization described above or rely on soil test results. Yes, adding additional fertilizer when it is not needed can cause a host of other issues. (see “Signs of over fertilization” section below)
When to fertilize azaleas – 2 times for the job
To avoid encouraging new growth late in the season that might be killed by early frosts, never fertilize azaleas in the late summer or early fall. Instead, opt for one of these two times: early spring to mid-summer or late winter. Let’s talk more about both of these options for when to fertilize azaleas.
When to fertilize azaleas – Option 1: In the early spring to mid-summer
Early spring is a great time to fertilize azaleas. Wait until the new leaf growth emerges to apply. Really, anytime from early spring to mid-summer falls under the optimum timing range for when to fertilize azaleas. Fertilizer is best absorbed when soil moisture levels are moderate to high. Check the forecast and feed azaleas before rain is expected or water it in with a hose after application.
When to fertilize azaleas – Option 2: In the late winter
Another possible time to fertilize azaleas is in late winter. This is particularly valuable if your primary objective is to acidify the soil. Since sulfur-based soil acidifiers take some time to effectively lower the pH, a late winter application means the pH change occurs sometime in the spring or early summer.
How to apply fertilizer to azaleas
Azaleas are shallow rooted, so there’s no need to work fertilizer deep down into the soil profile. Instead, sprinkle it on the soil surface above the root ball. Use a garden cultivator to lightly scratch it into the soil down to a depth of only 1 to 2 inches. The shallower roots of azaleas reach down only into the top few inches of soil. They can be damaged by deep cultivation. This is a simple way to provide azaleas with both the basic nutrients they need to thrive and also to adjust the soil pH if necessary.
How often do you fertilize azaleas?
Adding a second application or fertilizing azaleas every year is overkill and almost never necessary. New plantings can be feed during their second year. From there forward, test the soil every few years to determine whether or not more fertilization is even necessary. Focus on the soil’s pH so your azaleas can readily access the nutrients they need to reach their full potential.
Signs of over fertilization
There are a few issues that can become evident if you over fertilize azalea plants. New growth may become distorted, and leaves may brown/burn at their tips. Often, the most evident issues are pest problems. Insects and mites like scale, lace bugs, aphids, and spider mites are drawn to tender, over-fed foliage. If you provide too much nitrogen, an insect infestation is a common aftereffect.
While insecticides may temporarily fix the problem, reducing or even eliminating fertilization is essential for a long-term solution. Again, spend a few dollars for a soil test. They provide an incredible amount of peace-of-mind when it comes to knowing when to fertilize azaleas and when to hold off.
What to do after feeding azalea plants
After applying fertilizer to azaleas, mulch them with 2 to 3 inches of pine bark, pine straw, or another organic material. Pine-based mulches are ideal since they work to acidify the soil as they break down. Mulch also helps reduce weeds and stabilize soil moisture levels.
If you need to prune your azaleas do so only immediately after they bloom. Since azaleas produce flower buds in the late summer and fall for next year’s flowers, if you prune later in the season or during the winter, you’ll be cutting off all of the flowers for next year.
Do you need to fertilize florist azaleas in pots?
If your azalea is a florist’s azalea that’s growing in a small pot and was given as a gift at Mother’s Day or Easter, the plant most likely has a slow-release pelletized fertilizer already in the pot. There is no need to add more fertilizer. Florist’s azaleas are not typically winter-hardy varieties. They are forced to bloom out of season which requires a huge expenditure of energy on the part of the plant. Most gardeners consider these gift azaleas to be a disposable plant, and they toss them after the plant goes out of bloom. You can try to plant it out into your garden and see how it does, but chances are good that the plant will not survive the winter.
As you can see, knowing when to fertilize azaleas requires forethought and attention. It should not be an automatic process, but rather one that relies on data collected by a soil test. Again, this isn’t something you’ll need to do every year. However, if you want to see many more seasons of colorful blooms, it is worth the effort.
For more fertilization tips, please visit the following articles:
- Blueberry fertilization
- The best water-soluble fertilizers
- Peony fertilization tips
- Natural fertilizers for the garden
- What do fertilizer numbers mean?
- The basics of houseplant fertilization
Pin this article to your Garden Care board for future reference.