Questions and Answers on Successfully Growing Rhododendrons

Growing Rhododendrons is quite frustrating for many folks. Here's the deal in a nutshell. There are two kinds of these plants - the broadleaf (keep their leaves all winter) and the deciduous (drop their leaves). In general, the deciduous ones are hardier than the broadleaf.

Let me note that Azalea and Rhodendron are now lumped together as one plant rather than being separated in botanical classification.

Soil Conditions

They all prefer acidic soils. This means that the soil the roots are in (twice the width of the plant) should be acidic.

Too many gardeners expect to toss a little acidic fertilizer on the plants and have the plants thrive. This will work in the short term but in the longer term, an unhappy plant will simply slowly fade away over several years.

The solution is to acidify the entire bed using sulphur and peat moss as acidifying agents. And to monitor this acidity levels (somewhere down around 5.0 is good) on a yearly basis as the soil will tend to return to the acidity level of the surrounding soil.

There is some work that suggests that applying compost tea (made from a high quality compost and including leaf debris/mold) will enable rhododendrons to survive in regular soils. The bacteria added enable the plants to feed. I have not done this so can't really comment on how accurate it is but it is likely worth a try if you're having troubles.

Winter Hardiness

The most common species sold in garden shops is R. catawbiense and its hybrids so the following information is aimed at the general gardener and this plant. In general, the plant is bud hardy to -15F.

Some cultivars are hardier to lower temperaturs than this. So if your plant isn't blooming, and the winter temperatures got lower than this, you know the buds were frozen off.

Now plant hardiness is slightly colder than that with plants themselves surviving down into USDA zone 4. A hardier plant is 'Northern Lights' - this is a deciduous hybrid that is bud and plant hardy down to -45F.

I've grown these fragrant flowering plants and they are very good plants for beginning gardeners (and this advanced gardener too I note)


Here's where a lot of gardeners get themselves in trouble. This plant does not like full *winter* sunshine.

When exposed to the hot sun during winter months, the broad leaf species leaves warm up and then the stomata (sweat cells) open up allowing the leaf to lose moisture. The roots are in frozen ground so they are unable to replace this lost moisture.

The leaf browns off because of moisture loss.

Excessive wind will create a similar situation but the end result of both is a browning of the leaf edges or a complete browning of exposed leaf surfaces.

The plant will survive summer sunshine better than winter sunshine.

The ideal place is a location where the rhododendron is protected from the noon sun throughout the year.

Questions & Answers

What do I feed my Rhododendron? Compost. This is a woody plant - no more and no less - and it responds like any other woody plant to food. As long as you have the soil acidity right, then compost is an ideal food.

My Rhododendron has brown leaves, will it live? See the above note re sunshine location. Will it live? Darned if I know. Some times a plant can be defoliated two or three times and it will survive. Some times it only takes once. But as long as the stems aren't brittle and there's green under the tender bark (scratch with your thumb) then there's hope.

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How do I propagate my plant? Seeds work well for the most part. Cuttings are tricky; you're going to need rooting hormone, bottom heat, mist systems and 2-4 months from an August semi-hardwood cutting to get them to root up properly. You can try a spring softwood cutting but these are not usually reliable. If you're a home gardener, go with seed from your own plant.

What's the hardiest variety? 'Northern Lights' and they come in shades of orange and yellow and the flowers are sweetly fragrant. I grew this plant in my old garden (this new garden is on limestone so I rather doubt I'll be able to get the acidity necessary) and I loved it.

I got an azalea for Mother's Day -will it live outside?
These are florists azalea and are not considered hardy unless you live in a USDA zone 8 or so. Very tender plant but you can try. :-)

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