How to grow Rhododendrons in alkaline soils

Rhododendrons are arguably one of the most ornamental of all flowering shrubs, however they have a reputation for being a little difficult due to their soil requirements. The genus Rhododendron contains 1,024 species of woody plants, all of which display a requirement for growing in acidic soils with a pH of roughly 4.5-5.5. This is a strong characteristic of many members within Ericaceae family.

How to grow Rhododendrons in alkaline soils
When Rhododendrons are planted into unmanaged alkaline soils the roots of the acid-loving specimens become unable to absorb certain minerals, most notably iron and magnesium. Unable to take up these minerals results in a condition known as chlorosis. This is expressed in Rhododendrons as a uniformly yellowing of the leaves but with the leaf veins remaining green.

So if you have a passion for Rhododendrons, and are determined to grow ten in alkaline soils, what can you do to make this a success?

You have two options available, either create an isolated bed to plant your Rhododendrons into or adjust the soil conditions of the existing beds.

Isolated or raised beds

How to grow Rhododendrons in alkaline soils
Isolated or raised beds are relatively simple to create and easily fix the problem of planting into alkaline soils. Position the bed on top of a few inches of lime free gravel, then line the base of the bed with heavy duty plastic making sure that it has plenty of drainage holes to prevent waterlogging. Fill the bed with a soil-based ericaceous compost, alternatively create a 50:50 mix of finely-graded moss peat (a naturally acidic substrate) and sterilized top soil with John Innes base added to provide a suitable range of nutrients. Check the pH of the soil and if necessary soil can be made more acidic by the addition of sulfur, aluminum sulfate, or iron sulfate. Off the shelf products such as Murphy's Sequestrene (chelated iron) can help with this.

Adjusting soil conditions

Digging in plenty of finely-graded moss peat will help to improve the existing soil but by itself can only be considered a temporary measure. Dig in small but regular amounts of sulfur, aluminum sulfate, or iron sulfate to acidify the soil further, checking the pH levels a couple of weeks after each application.

Moving forward....

To maintain optimum conditions, add sulphur regularly, only water with rainwater and feed every few weeks over the growing season with Murphy's sequestrene

Using science!

Rhododendron Inkarho -
There is a general rule that states that it is always easier, and usually better, to have plants that fit the environment rather than to make the environment fit the plants. However there has been a significant development in the cultivation of Rhododendrons with the development of the Inkarho rootstock.

The Inkarho rootstock story began when a self-seeded rhododendron was spotted growing in a quarry well-known for its limey soil and bedrock. This was then bred with Rhododendron 'Cunningham’s White’ producing approximately 20,000 seedlings. These were grown to maturity and tested for lime tolerance. Eventually the best performing specimen was selected and given the Inkarho name.

The Inkarho specimen was not a particularly ornamental plant, however it has served to be suitable rootstock for existing cultivars. Grafting compatible Rhododendron cultivars onto the Inkaro rootstock enables the plants to thrive in alkaline soils with a pH of up to 7.5 There is also evidence to suggest that they will also improve the cold tolerance of these grafted plants to as low as - 20 degrees Celsius).

Main image credit - 松岡明芳 - 松岡明芳 CC BY-SA 3.0
In-text image credits - Kirin X CC BY-SA 2.5, Trivia King CC BY-SA 3.0

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