Hydrangeas and Rhododendrons – Similarities & Differences

Two plants that firmly find a place in the category of the perfect flowering garden plant are Hydrangeas and Rhododendrons. Both plants will add splashes of green foliage as well as add color with their flowers. 

To anyone who has never heard of these plants before, you may ask what are the similarities and differences.

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Similarities between Hydrangeas and Rhododendrons

Hydrangeas and Rhododendrons stand out from the crowd of plants at the nursery because they both color the garden with their unique flowers. Also, they are both easy to propagate, even for novel gardeners!  

1. Ease of Growing

Did you know you can grow hydrangeas and rhododendrons from just stem cuttings? Yes, you can from stem cuttings! Take the stem cuttings before the fall or early spring, it gives the cuttings time to establish roots before the blooming season.

Take a firm stem with two sets of leaves, you should remove the bottom leaves. You can use a rooting hormone, cinnamon is a good natural rooting composite. Dip the end of the stem in the cinnamon, and pot up the stem. You will see the new growth in a few weeks.

2. Soil pH

Both rhododendrons and hydrangeas have similar needs when it comes to their soil pH and types of soil. They both require a pH of 5 and 5.5 although hydrangeas can tolerate a slightly higher pH. 

The color of the hydrangea varies depending on the acidity or alkalinity of the soil. If the soil is acidic you will find your hydrangea with blue blooms, and pink blooms if the soil is alkaline. If the soils are neutral, the color could be white or pale green.

On the other hand, the rhododendron’s color is not affected by the pH of the soil, although they do better with slightly acidic soil.

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3. Fertile Soil

Both plants enjoy soils that are rich in organic matter but must be free draining. Whether you provide nutrients in the form of fertilizer or compost, both plants won’t really mind! 

In addition to compost-rich soil or a bout of fertilizer after winter or early spring, you can give these plants a 1-2 inch thick layer of mulch to help retain moisture and leach out nutrients at intervals of time. 

A lack of nutrients will affect the growth of both plants. Also, when talking about these two plants that bloom well, poor nutrient availability can interrupt blooming. Flowers will be sparse or they may not flower at all! 

4. Moist But Not Wet Soil

The hydrangea and rhododendron require well-draining soils that do not keep water around their roots. Waterlogged soils can kill your plant through root rot, soils need to be moist but not soggy,

Hydrangeas can benefit from a bit of fertilizer, to make up for lost nutrients, but if your rhododendron is looking good and healthy, leave it as is. What you can do is give both plants a mulch, to see a significant improvement in blooms.

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5. Full to Partial Sun

Both of these plants are not suited for full shade or even more shade than sun. If given more shade than the sun, they will not grow well and blooming may not occur. That said, you will have to place them in full sun or partial shade to get them to grow and flower to their full potential. 

Be sure to put your hydrangea and rhododendron in a spot that gets enough sunlight. Note that you have to keep variegated varieties in a place with not as intense sunlight to prevent fading. Signs of lack of sunlight could be yellowing of leaves and droopiness as well. 

6. Toxicity 

Another quality that hydrangea and rhododendron share is that they are both toxic. All parts of these plants have chemicals that can cause harmful effects on humans, cats, dogs, horses, and even birds!

Another problem with rhododendron is that nectar collected from their flowers can be collected by bees, resulting in mad honey. ‘Mad Honey’ is honey with a high concentration of chemicals that are toxic. When consumed, these chemicals could affect any living creature.

If consumed in very high amounts, it could even be fatal!

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Differences between Hydrangeas and Rhododendrons

While there may be quite a lot of similarities between these two plants there are a few differences. These differences are what sets them apart and make you decide if you can grow them together or not. Take a look at where these plants differ and how this affects their ability to grow together.

1. USDA Zone Hardiness

Rhododendrons will do well in zone 8 (10℉ – 20℉), and hydrangeas do best in zones 9 to 10 (20℉ – 40℉) These are warmer climates and these plants are not very cold hardy at all. You will have to bring them indoors if growing in locations where the temperatures are much lower than what they can withstand. 

While these are the most common zones for these two plants, there are new cultivars that can do with even lower temperatures. The French hydrangeas can survive in zones 6 to 10 (-10℉ – 40℉). The Oakleaf hydrangea prefers zones 5 to 9 (-20℉ – 30℉).

The rhododendron Florida Pinxter does well in zones 6 to 9 (-10℉ – 30℉), and the peasant white will grow in zones 5 to 8 (-20℉ – 20℉). 

2. Watering Habits

Hydrangeas need an inch of water per week in the growing season. To encourage a good root system, you should water deeply. Water in the morning when hot, and try not to water the leaves, as it can be a cause of sunburn.

Rhododendrons need to be watered when it’s cool in the morning, and at the base of the plant. For mature plants one inch of water every two weeks is fine, unless the weather is very hot, in which case, more water is required.

Rhododendron does not need as much water as hydrangeas, so this needs to be noted if you don’t want to accidentally kill one of the plants!  

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3. Plant Spacing 

When planting rhododendrons leave a space of 48 inches apart and plant at a depth of 18 inches. If you find that you might have problems when it comes to drainage, you can plant both plants in raised beds.

Hydrangeas do need more spacing as their foliage does tend to spread out quite a bit. Providing it with enough space does mean that moisture under and around the foliage can dry up. Failure for excess water to dry up can cause disease and induces fungal growth. 

Space your hydrangeas 36 to 60 inches apart, and the planting hole should be 2 inches wider than the plant’s root ball.

4. Bloom Colors & Frequency 

Hydrangeas are well known for their wonderful pink and blue blooms, but did you know that they come in a range of colors from pale yellows and whites to pinks, violets, and blues. There are even oranges and reds, however, all this depends on the soil pH and cross-breeding!

Rhododendrons too come in an amazing array of colors, such as orange, red, deep red, yellow, white, cream, pink, and purple. Additionally, you can even get your hands on specialty bred varieties that can assure several reblooming efforts in the flowering season. 

Not to mention that hydrangea and rhododendron flowers look very different. They feature different sizes and also the shape of the flowers varies greatly and this is not a bad difference. Instead, it gives great depth to the garden by providing different shapes and shades. 

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For anyone looking for flowering plants that will beautify the garden with extraordinary blooms and colors, hydrangea and rhododendron come to mind. 

While there are many similarities between these plants, there are also a few that distinguish them as very different plants. 

Similarities- Easy to grow, Soil pH, Nutrient requirements, Soil moisture, Full-sun/Partial shade, High toxicity. 

Differences- USDA growing zone, Watering habits, Plant spacing, Bloom colors, and frequency. 

Despite these differences, these plants can actually be grown together or nearby as long as they are equally hardy. Also, you will have to water these plants separately to not kill off one of them! 


How can you tell the difference between Rhododendron and Azalea?

The shapes and sizes of their flowers are a true way to tell the difference between rhododendron and azalea. Rhododendrons have bell-shaped flowers that appear in clusters and are considerably larger than the azalea flowers. Azalea flowers are much smaller and take up a more tubular or funnel-shaped form.  

Can you plant Rhododendron and Hydrangea together?

Yes, you can plant rhododendrons and hydrangea together. Most of their requirements are the same, making them good companion plants. However, the biggest difference between these two plants is that they have very different watering habits. Giving one too much water or too little will lead to one of the plants dying!