Rhododendron Identification: And Why Every Part Is Toxic

Rhododendron or azalea

Azaleas are rhododendrons, botanically. They sit inside the same genus. But gardeners separate them because they are grown differently and they look different, and the separation is useful.

The most reliable single difference is the stamens. Rhododendron flowers typically have ten or more stamens. Azalea flowers typically have five. Pull one flower apart and count — it takes ten seconds and it settles it.

The second difference is the leaf. Rhododendrons are usually evergreen with large, thick, leathery leaves. Azaleas have smaller, thinner leaves that often feel slightly bristly, and deciduous azaleas lose them entirely in winter.

The third is how the flowers are held. Rhododendrons hold their flowers in trusses of ten or more, a big ball of bloom. Azaleas hold them in smaller, looser clusters.

The groups you are likely to meet

What to look at on the leaf

• The underside. Many rhododendrons have a distinctive felted or powdery coating underneath, called indumentum. It ranges from silver to rust red and it is a strong identifier.
• The leaf edge. Most rhododendrons have smooth, entire edges. Most azaleas have finely toothed or bristly edges.
• The shape. Large, long and leathery points to a rhododendron. Small, oval and glossy points to an evergreen azalea.
• The new growth. Pieris has distinctly bronze or red new growth in spring, which rhododendrons do not.
• The buds. Rhododendron buds are large and covered in overlapping scales, and they are visible all winter.

The toxicity, plainly

Rhododendrons and azaleas contain grayanotoxins in every part of the plant — leaves, flowers, nectar, pollen and honey. Eating them causes vomiting, diarrhoea, a slow heart rate and low blood pressure, and severe cases need hospital treatment.

The honey point is the one people find surprising. Honey made by bees working rhododendron flowers can contain enough grayanotoxin to cause illness, which is the origin of the historical "mad honey" poisonings. Commercial honey is blended and tested, so this is not a reason to worry about a jar from the shop. It is a reason not to make honey from a rhododendron thicket.

For pets, the leaves are the risk, and they are fallen leaves as much as growing ones. A dog that chews a branch can get a meaningful dose.

The plant is not dangerous to touch. It is dangerous to eat. That is a distinction worth keeping clear.

Growing one, and why people fail

Almost every failed rhododendron is a soil problem. They need acid soil, around pH 5 to 6, and they will not tolerate lime. In a neutral or alkaline garden they go yellow, stop growing and eventually die, and no amount of feeding fixes it.

They are also shallow-rooted, and they dry out faster than their size suggests. Mulch them, water them in dry spells, and never cultivate around the base.

Plant them in spring or early autumn, in dappled shade with shelter from cold wind. The flowers and buds are frosted easily, and a late frost will take the whole season.

If your soil is alkaline, grow them in a large container in ericaceous compost. That is genuinely the only reliable answer, and it works well for the smaller varieties.

There is a garden near me with a forty-year-old rhododendron that is now the size of a small house, and every spring someone asks me what it is and whether they can plant one. The answer to the second question depends almost entirely on their soil.

But before any of that, there is a safety point that does not get said often enough. Every part of a rhododendron is toxic — leaves, flowers, nectar — and honey made from rhododendron nectar can be toxic too.

That does not make them dangerous to have in a garden, in the way that, say, foxgloves are not dangerous to have in a garden. It does mean you should know it before children or dogs start chewing the leaves.