Rhododendrons are a fantastic option for those looking to add pops of color in the summer and volume to their garden year-round. With over 1,000 different species of rhododendrons available, you’ll have plenty of variety to choose from. To give your Rhododendron the best chance to thrive, you’ll need to consider the soil, sun, space, and climate.
Rhododendrons do best when planted in well-draining, acidic soil, moderate sun with dappled shade, in an area between 2 to 6ft (0.61 – 1.83m), and climates with moderate rainfall. Experts indicate that rhododendrons do best in USDA hardiness zones 4 through 8.
The rest of this article will go into greater detail about rhododendron characteristics, how choosing the right space, and giving it the proper care will allow your Rhododendron to grow healthy and strong for many years to come.
Rhododendrons originated in Southeast Asia and were first discovered in the 16th century by botanist Charles l’Ecluse. The Rhododendron itself is classified as a genus in the heath family of plants.
There is much variety within the rhododendron genus, and plant species within the family can be evergreen or deciduous, meaning they can thrive in both warmer and cooler climates.
Rhododendrons are known for their broad, green leaves and rich colored flowers – ranging from white and yellows to deep red and purple hues. Rhododendrons are an invasive plant that can grow indefinitely if not pruned seasonally (the largest Rhododendron on record currently stands at 108 ft [32.9 m] and is still growing!).
Rhododendrons prefer moist, well-draining soil with a pH level between 4.5 and 6.0 (which means slightly acidic) that is composed of rich, organic matter. Keep reading for more information about each of these sub-categories.
Because of their baby-fine root systems, rhododendrons are prone to root-rot, a disease in plants that causes roots to decay due to mold and lack of aeration if the soil cannot properly drain. With that being said, the soil still needs to be moist for rhododendrons to succeed in their environment.
How to Test Your Soil
To strike the perfect balance between well-drained yet moist soil, you’ll want to add a loose substance that can retain moisture to your garden areas like compost. The mixture of compost and native soil allows water to pass through and hold on to moisture without water-logging the roots.
To test your soil for potential drainage issues, start by digging a hole 10 to 12 inches (25.4 to 30.1 cm) and then fill it with water. Once it drains, fill it with water again and take note of how long it takes for the water to seep into the ground.
If the water has dissipated within an hour, your soil is well-draining. If there is still some water left in the hole after the hour is up, your soil is not well-draining, and you’ll need to take steps to correct that before planting your Rhododendron.
If your soil isn’t well-draining, there are still steps you can take to adjust it. A raised bed of organic soil about 12 to 18 inches above the native soil in your area will allow for the roots of your Rhododendron to establish while ensuring that the soil will drain properly.
Be sure to secure the raised bed in place with some sort of wall or barrier around the soil, so it doesn’t make a mess. Raised beds tend to dry out more quickly in the summer, so be sure to water frequently.
Rhododendrons require acidic soil so that they can absorb the nutrients it provides them. Rhododendrons benefit most from the nutrients nitrogen (N), phosphorus (P), and potassium (K), and acidic soil allows for those nutrients to absorb more efficiently.
How to Test the Acidity of Your Soil
To test your soil for acidity, scoop about a half cup of soil into a bowl and then add some vinegar to it. If the mixture of soil and vinegar bubbles, that is an indication your soil is more alkaline and has a lower pH.
If your soil mixture does not bubble when the vinegar is added, take another sample of soil and add it to a second bowl, and then mix some baking powder into it. If the soil mixture bubbles and fizzes with this test, that will indicate your soil is more acidic.
If there is no reaction, your soil is likely neutral and will require acidity to be added to it. You can add acidity to your soil by adding sulfur, pine needles, or sphagnum peat to it. You can purchase fertilizer meant for raising acidity levels at your local garden center.
Rhododendrons do best when planted in areas that are moderately sunny but still receive dappled shade throughout the day. You’ll want to avoid areas where there is too much shade or sun, as either can stunt your Rhododendron’s growth.
Picking the Right Location
Rhododendrons do not like full morning sun but will benefit from 6-8 hours of moderate sun in the afternoons. Typically, the north side of a building or home will meet these criteria. However, if you don’t know which side of your house faces north, there are still some things you can do to determine the best place for your Rhododendron.
Observe the prospective planting areas in your yard at different times throughout the day. What time does the shade cover disappear, and how long does it remain sunny? If you can find a spot that has at least 6 hours of partial sun, that will make a good spot for your Rhododendron.
Understanding what your rhododendron needs for space requires an understanding of what variety of Rhododendron you’re planting, and how big the plant will be at maturity. Most plants will disclose the estimated size your plant will grow to right on the tag.
Measuring Your Space
Observe the mature height and width of your Rhododendron and mark off that width in your garden. Dig a hole at least 2 feet deep, and then be sure to leave at least 12 inches (30.5 cm) of space on all sides to allow your Rhododendron to grow uninhibited.
Adding Rhododendrons to an Existing Garden
If adding a rhododendron to an existing garden, plant it near other plants that require the same growing conditions as a rhododendron. Some examples of plants that succeed in the same conditions as rhododendrons are Muscari, narcissus, and bleeding hearts.
Rhododendrons do best when planted in USDA hardiness zones 4 – 8 that receive moderate rainfall. These zones represent areas that have a minimum low temperature of -30°F (-34.4°C) in zone 4, up to 10°F (-12.2°C).
In summary, rhododendrons do best when planted in the following conditions:
- An acidic, moist, well-draining soil
- Partial shade, moderate sun
- In areas with other acid/shade-tolerant plants
- In areas large enough to allow the Rhododendron to grow to their mature size uninhibited
If you have a spot that meets most, but not all of the criteria above, there are things you can do to adjust it so you can ensure your Rhododendron will do well there. You can add acidity to the soil if it’s too alkaline, you can add a raised bed if your soil doesn’t drain well enough, and you can move other plants around to allow room for your new Rhododendron.