Rhododendrons are a beautiful addition to any garden. With their lush foliage and large, colorful blossoms, rhododendrons are one of the most popular options for an eye-catching landscape. Fertilizing your rhododendrons will keep the soil prosperous and ensure your blooms return vibrantly, year after year.
To fertilize rhododendrons, you’ll need to choose a fertilizer, apply it to the soil around the base of the plant, and water it generously. Fertilization should occur in late winter or early spring, after the final freeze of the cold months.
The rest of this article will explain how to properly fertilize rhododendrons, why rhododendrons don’t need to be fertilized often (if ever!), and what signs would indicate your plant needs fertilizing. I will also share some information about using mulch to naturally fertilize your plants.
1. Evaluate Whether Your Rhododendron Needs Fertilizing
Surprisingly, rhododendrons don’t usually need to be fertilized. Fertilization is only necessary for a few circumstances where the plant needs extra nutrients due to poor soil quality or adverse weather conditions.
You can determine whether your rhododendron needs fertilizing by checking for any indications of malnourishment. Some of these indicators are:
- Slowed growth
- Yellowing leaves
- Lack of blooms
If your rhododendron bush exhibits any of these telling symptoms, there is a good chance that your soil is deficient in one or more essential nutrients and would benefit from a fertilizer treatment. Rhododendrons need nitrogen, phosphorus, and potassium to grow, so when one or more of these nutrients are unavailable, the plant struggles to thrive.
There are a number of factors that can cause your soil to be deficient in essential nutrients. Some of the factors that can impact the quality of your soil are:
- Living in a dry climate. Rhododendrons like moisture, so living in a dry environment can be stressful for them. If you have a dry season, especially in the winter, it can cause stress to the plant, impacting its coloring and ability to bloom.
- Mulching with sawdust or wood chips. sawdust and woodchips use up a lot of nitrogen as they mix with the soil. If you regularly mulch with these materials, it is likely that your soil is low in nitrogen; an essential nutrient for the health of your rhododendron.
To determine if there are any nutrient deficiencies in your soil and to identify what those deficiencies are, I recommend taking a soil test. A soil test evaluates your soil’s overall quality and content and indicates any components that are lower or higher than they should be.
2. Choose the Best Rhododendron Fertilizer for Your Plant
Once you have evaluated your rhododendron bush and have conducted a soil test, you should have a pretty clear idea of what your soil needs to best support your plant.
There are many different types of fertilizer available, but it is vital to choose the right one for your needs, as rhododendrons are extremely sensitive to fertilizer and may not react well if you choose the wrong one.
Some of the best fertilizer options for rhododendrons are:
- All-purpose fertilizer. Most commonly, a low dose of a well-balanced, all-purpose fertilizer will do the trick. With a healthy, even ratio of nitrogen, phosphorus, and potassium, your rhododendron bush will be well supported.
- Rhododendron and azalea fertilizer. Fertilizers made specifically for rhododendrons are a great choice because it takes into account the unique needs of this flowering plant. I recommend Dr. Earth’s Organic Rhododendron Acid Fertilizer (available on Amazon.com) because it includes specially blended soil microbes that support maximum flowering.
- Coffee grounds. Coffee grounds have been found to be an excellent, all-natural source of fertilization that rhododendrons love! Save your coffee grounds and reuse them to improve the health of your plants!
- Nutrient-focused fertilizer. If, after conducting a soil test, you discover that your soil is rich in some nutrients but very low in others, you may want to choose a nutrient-specific fertilizer. For example, if you have nitrogen-deficient soil but everything else is balanced and well-supplied, you may want to consider adding a high-nitrogen fertilizer.
3. Carefully Measure the Right Amount of Fertilizer
The right amount of fertilizer for your rhododendron bush is dependent upon the size, age, and health of the plant. It is absolutely critical not to over-fertilize your rhododendron plant as they are extremely sensitive. Too much fertilizer can result in damaged roots and leaves and could even cause the plant to wither and die.
Most fertilizers will come with clear instructions for the amount recommended for use. Be sure to read the manufacturer’s recommendations thoroughly before applying any of the fertilizer to your soil.
Using an organic fertilizer is an excellent choice for rhododendrons because it works more slowly than synthetic fertilizers. A slower distribution of nutrients will reduce the risk of “shocking” the plant with too much at one time.
Typically, you’ll want to follow these guidelines when measuring out fertilizer for large-leaved rhododendrons:
- Plants 12-24 inches (30-60 cm) high need about 1.17-1.76 ounces per square yard (39.7-59.7 grams per square meter).
- Plants 28-47 inches (71-120 cm) high need about 3.17 ounces per square yard (107.5 grams per square meter).
Older, well-established Rhododendrons could benefit from slightly higher amounts of fertilizer, up to 4.23 ounces of fertilizer per square yard (143.4 grams per square meter).
If you are working with a smaller-leaved variety of rhododendron, you should note that these plants require only about half of the above recommendations.
Regardless of how malnourished your rhododendron plant is, you must not exceed the amount of fertilizer recommended. Using more fertilizer than the appropriate dose will not improve or speed up your results–it will actually have the opposite effect, causing more harm than good.
4. Sprinkle the Fertilizer on the Soil at the Base of the Plant
To apply fertilizer to your rhododendrons, you’ll want to focus on the “crown” at the base of the bush. The crown is basically a border surrounding the bottom of the plant outside of the roots.
If you have any mulch on top of your soil, you will need to create a shallow trench all the way around the plant before applying the fertilizer. If fertilizer is applied to the top of a layer of mulch, it will first be absorbed by the mulch rather than the soil. When this occurs, so many of the fertilizer’s nutrients are used up by the mulch that there’s barely anything left for the roots of the rhododendrons.
When digging your trench, take care not to dig into the soil. Rhododendron roots are delicate and very sensitive. They tend to run near the top of the soil, so if you start digging into the soil around the base of the plant, you could disturb or damage the root system.
After carefully measuring the appropriate amount of soil for your plant size, you will apply it to the base of the plant. Gently sprinkle the fertilizer into the trench that you created with your mulch, or if you don’t have any mulch, sprinkle it directly onto the soil.
If you have used the trench method, you will need to move the mulch back into the trench, covering the fertilizer and leveling out the mulch at the base of the plant. If you have applied the fertilizer directly to the soil, all that’s left to do is water it in.
5. Water the Fertilizer Into the Soil
Once the fertilizer is applied, it must be watered in. Granular fertilizer needs water to help it release nutrients, soak in and integrate with the soil. If left without water, it will simply sit on top of the soil, providing no nutrition to the plant whatsoever.
Water encourages the fertilizer granules to start breaking down and releasing their nutrients into the soil. It also softens the ground, making it easier for the fertilizer to break through the surface and integrate with the roots of your rhododendron.
It’s always a good idea to keep an eye on your local weather forecast when planning to fertilize. It is ideal to fertilize prior to a forecasted rainfall so that the fertilizer can get watered in evenly and naturally.
If you are fertilizing on a dry day, you will need to make sure to manually water the fertilizer in, either with a spray nozzle on your garden hose, a sprinkler system, or a watering can.
It is also a good idea to water the soil deeply and thoroughly a day or two before fertilizing to prepare the soil for the acceptance of the added nutrients.
Liquid fertilizer can have its own requirements. I’ve written another guide to discuss the ideal time to water liquid fertilizer. Read it to learn more: How Long Should You Wait to Water Liquid Fertilizer?
When Should You Fertilize Rhododendron Bushes?
The recommended times to fertilize rhododendron bushes are:
- Late winter
- Early Spring
- Late Spring
- Early Fall
Let’s take a look at the guidelines for fertilizing during each of these seasons.
Fertilizing Rhododendrons in Late Winter
When fertilizing rhododendron bushes in the late winter, it is essential to make sure that you have passed through the final freeze of the colder months. Fertilizing too early can cause harm to your rhododendrons should an unexpected freeze occur after your first application.
Once the final freeze has thawed and you are heading into the warmer months, it is usually safe to start applying fertilizer. Your plant will welcome the extra nutrients after a cold winter, especially if it is a particularly dry one.
Fertilizing Rhododendrons in Early Spring
Early spring is perhaps the most ideal time to fertilize a rhododendron bush. With the threat of a surprise freeze behind you and the warm summer months ahead, spring is a pretty mild, safe time to fertilize.
By fertilizing during the spring months, you are replenishing nutrients that were potentially lost during the cold winter and boosting your plant’s health before the heat of a warm summer.
Fertilizing Rhododendrons in Late Spring
Some gardeners insist that fertilizing your rhododendrons in late spring, just after the final bloom, is the best time to fertilize. By fertilizing just after the last bloom, the rhododendron is able to replenish enough nutrients to support more robust, vibrant growth for the next blooming season.
If you choose to follow this method, it is important not to fertilize past the end of June. Fertilizing into the summer months will encourage your plant to produce new, lush growth, only to be severely damaged by the cold weather heading your way between fall and winter.
Fertilizing Rhododendrons in Early Fall
If you would like to give your plants a boost of extra nutrients before the bitterly cold weather blows in, you can fertilize your rhododendrons safely and organically by using mulch. In contrast to the granular fertilizer method previously shared, mulch offers health-boosting nutrients, conserves moisture, and provides root protection from the cold.
Why Rhododendrons Don’t Usually Need Fertilizer
When planted in rich and fertile soil, rhododendrons really don’t need any fertilizer to thrive. Because rhododendrons are so sensitive to changes in soil, they are highly susceptible to over-fertilization. Too much fertilizer can cause more harm than good, so it is often recommended to leave them alone if fertilizer needs are not seriously apparent.
Much like other plants, rhododendrons need nitrogen, phosphorus, and potassium to grow. They also need small amounts of:
- Magnesium
- Iron
- Calcium
- Boron
- Manganese
- Copper
- Zinc
- Molybdenum.
Rhododendrons are acid-loving plants and so require a lower pH level soil. All of the nutrients needed to sustain rhododendrons can be found naturally in most climates. They thrive particularly well in areas that maintain a mild climate with good levels of moisture, like its home state of Washington.
Signs Your Rhododendron Needs To Be Fertilized
Although rhododendrons don’t often need to be fertilized, there are some cases in which their nutrient deficiencies are causing growth issues. Under these circumstances, a helpful, mild fertilizer can be applied to help your plant regain total health.
If your rhododendron bush has slowed or stopped producing blooms altogether, is showing discrepancies in coloring (i.e., yellowing, brown spots, fading, etc.), or has thinning foliage, you should probably apply a rhododendron-appropriate form of fertilizer. This will help correct any nutrient deficiencies and restore health to the plant.
Before you rush out to the store for a special rhododendron fertilizer, first make sure that your plant’s needs are being met by evaluating the following:
- Is your rhododendron bush getting enough sunlight?
- Are you watering your rhododendron bush regularly and sufficiently?
- Did you prune your rhododendron too much or improperly?
If any of these questions point out some potential deficiencies that you can correct without fertilizer, try to remedy the issue first before you apply fertilizer treatments. As stated, too much fertilizer can cause more harm than the good intended, so if it is unnecessary, you shouldn’t use it.
Using Mulch To Fertilize Your Rhododendrons
You can fertilize your rhododendron plants naturally with mulch in addition to, or instead of, granular fertilizing. This is a great option because it is slow-acting and completely organic, reducing the risk of adverse reactions to the sudden increase in nutrients.
To mulch your rhododendrons, you can use a variety of options. Here are some examples, to name a few:
- Sawdust
- Fresh wood chips
- Rhododendron mulch blend
- Compost
When applying the mulch to your rhododendrons, you’ll want to lay a light, even layer, all around the plant, leaving a bit of space at the trunk of the bush. It is crucial for rhododendrons to have some breathing space, so you don’t want to smother the bush with too much mulch.
For the same reason, using any type of landscape fabric or weed cloth underneath the mulch layer is not recommended. While it is appealing for the potential of eliminating weeds, it can cause problems with the roots of the bush.
Because rhododendron roots are so thin and delicate near the surface of the soil, too much pressure can cause issues. They also need access to oxygen, which can be cut down too much with the addition of weed cloth.
It is important to note that if you choose to use sawdust or fresh wood chips as mulch, they will almost certainly reduce the amount of nitrogen in the soil, an essential nutrient for the growth of rhododendrons. The likelihood that you will need to lay a nitrogen-rich fertilizer to balance your soil’s nutrients back out is far greater when using these types of mulch.
If you do use sawdust or wood chips as mulch and end up needing to add nitrogen to your soil, check out my other article about additives that increase the nitrogen in the soil for some helpful tips: 10 Things That Add Nitrogen to Soil
Final Thoughts
Fertilizing rhododendrons is a relatively quick and straightforward process but is only necessary when there is a true nutrient deficiency in the soil. Rhododendrons usually do very well without fertilization if they are planted in rich, healthy soil.
Rhododendrons can be fertilized with a rhododendron-specific fertilizer formula or an all-purpose, balanced garden fertilizer. Because they love acidic environments, the fertilizer needs to support a low soil pH.
Fertilization should occur in late winter or early spring, and mulching for extra protection can be done in the early fall before the cold of winter begins.
