Are Rhododendrons and Hydrangeas the Same? (Explained)

Rhododendrons and hydrangeas are completely unrelated plants with very different appearances, but they do share some traits in the growing conditions they prefer. For instance, they both prefer acidic soil and a good mix of sun and shade. Both look spectacular in full bloom, but have flowers of varying sizes, shapes and colors.

Soil

Both hydrangeas and rhododendrons are sensitive to soil pH although rhododendrons are more sensitive.

They both prefer acidic soil with a pH level between 5 and 5.5. However, being less sensitive, hydrangeas can tolerate soils with a higher pH and will often still do well in soil that is slightly alkaline.

Rhododendrons on the other hand won’t.

They both also like soil that is consistently moist, without being boggy, and which is rich in organic matter.

If you have soil that doesn’t drain well neither plant will be happy, with hydrangea possibly being the more sensitive and susceptible to root rot.

The pH level in the soil can change the color of hydrangeas flowers:

Soil pH of 5.5 or lower: blue flowers
Soil pH of 5.5 to 6.5: purple flowers
Soil pH of 6.5 or higher: pink flowers

The color of a rhododendron’s flowers is unaffected by soil pH.

SIMILARITIES: Both prefer acidic soil, both like moist well-drained soil
DIFFERENCES: Hydrangeas are more tolerant of alkaline soil, the color of hydrangeas flowers can change according to soil pH.

Water

The ‘hydra’ part of a hydrangea’s name here gives us a hint that they need more water than rhododendrons.

Hydrangeas require about an inch of water per plant, per week in the growing season.

Rhododendrons can get by with just a trickle of water from the hose, with one inch every two weeks being sufficient.

As noted above, both like well-drained soil that is kept moist. Too much water in the soil and both are at risk of root rot.

DIFFERENCES: Rhododendrons do not need quite as much water as hydrangeas.

Sun and Shade

Hydrangeas and rhododendrons are similar in that they both like a mix of full sun and some shade.

Hydrangeas like the sun in the mornings when it is cooler and then shade in the afternoon when the sun is at its hottest.

There is a bit more variance when it comes to rhododendrons, with some varieties able to tolerate full sunshine. However on the whole most rhododendrons are happiest in sun-dappled shade.

Neither plant will do well if left in complete shade all the time.

SIMILARITIES: Both like a mix of sun and shade and won’t grow well if left in complete shade.

Flowering

Both rhododendrons and hydrangeas look spectacular when they are in full bloom.

Hydrangeas tend to bloom from mid-spring right through to early fall, whereas rhododendrons flower from late winter to early summer.

Hydrangeas tend to flower longer than rhododendrons through the summer.

The flowers of the respective plants look quite different, varying in size and shape.

Hydrangeas flowers are large round globes or flowerheads, whereas rhododendrons sport bell-shaped flowers.

Both provide you with a smorgasbord of color.

Hydrangeas are most commonly associated with being pink and blue, but they can also be yellow, white, purple, violet, orange, and red.

Rhododendrons too come in a wide array of colors, including orange, red, yellow, gold, white, cream, pink and purple.

SIMILARITIES: Lots of beautiful vibrant flowers.
DIFFERENCES: Slightly different blooming periods, hydrangeas flower for longer in the summer, different shape and size of flowers.

Size

Both plants are fast growers and both can grow very big.

Hydrangeas can grow up to 15 feet in height and rhododendrons up to 20 feet.

They both vary wildly depending upon the variety.

SIMILARITIES: Both are fast growers and can grow big.

Feeding

Neither hydrangeas or rhododendrons are fertilizer-hungry plants.

Hydrangeas probably can benefit from fertilizer slightly more. A dose in the spring can help them build up nutrients they may have used over the winter months.

If your rhododendron looks healthy and is growing well you can leave it be.

Both will benefit from a layer of compost or mulch however.

SIMILARITIES: Neither are fertilizer hungry.
DIFFERENCES: Hydrangeas may benefit from a dose of fertilizer in the spring, whereas rhododendrons tend to be fine without it.

Deciduous or Evergreen?

The rhododendron genus is essentially evergreen.

Hydrangeas generally die back in the winter, although they can be evergreen in warm climates.

DIFFERENCES: Rhododendrons are usually evergreen, hydrangeas are usually deciduous.

Pests and Disease

Both plants are reasonably maintenance-free when it comes to pests and disease.

Hydrangeas can be bothered by aphids and spider mites occasionally, with powdery mildew also a problem sometimes.

The most common problem for rhododendrons is a lack of water in the fall can lead to sun scald during the winter. On occasion, iron deficiency can turn the leaves yellow.

Both might experience root rot if the soil is too wet, due to their aversion to boggy soil.

Final Thoughts

So as you can see whilst hydrangeas and rhododendrons aren’t the same, they do share a number of similarities.

Not least in the type of soil they thrive in and their propensity for sun and shade.

They work well together as a pairing, with the vibrant color of hydrangeas working well in beds, with the slightly bigger rhododendrons behind.

One area where they are different is in the shape and sizes of their flowers.

This gives them another advantage as a pairing with a variety of shapes, shades and sizes giving great depth to a garden when used together.

They are both gardening show stoppers and will make any garden look wonderful, whether used together or in isolation.

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