The Tree Center


Written by Dave Gs • October 23 Growing Rhododendrons on Alkaline Soil

Rhododendrons, and Azaleas too, are loved by everyone for the brilliance and variety of their spring flowers. Nothing rivals the beauty, range of colors, and abundant display of these plants. Among many people they have an undeserved reputation for being difficult to grow, mainly because of their requirement for acid soil. But beyond that they are easy – just keep the soil damp, and perhaps feed them in spring. They have few if any serious pests. There are varieties that will grow in both the coldest and the warmest parts of the country. Perhaps best of all, they are happy in those areas of partial shade that gardeners are always looking to fill.

Rhododendrons Love Acid Soil

Across much of the east, from Maine to Florida, the soil is naturally acidic, so growing these plants in the acidic soil they need is not a problem. In most other parts of the country things are not so easy. Rhododendrons ideally need soil with a pH of 6.0 or less. These plants cannot absorb the iron they need from the soil in alkaline soils, and this shows as poor growth, and especially as a yellowing of the new leaves in spring. This can be slight, or very severe, with the leaves appearing completely golden in color. This is called ‘chlorosis’, and rhododendrons suffering from this condition will grow very slowly, flower very little or not at all, and in the long-term, languish and die.

Growing Rhododendrons in Pots

One obvious solution if your garden soil is not acidic is to grow these plants in pots. There, it is easy to provide suitable soil, available as ‘acid-loving plant soil mix.’ The pot should have a drainage hole, and use one that is about twice the diameter of the pot your plant is already growing in. Rhododendrons have a fibrous, shallow root system, so the pot does not have to be very deep. In fact, shallower pots are better, especially for smaller plants. When potting your new treasures, make sure you plant them at the same depth they were in the original pot. Cover the root ball with only a very thin layer of the new soil, or leave it completely exposed.

Rhododendrons will grow for years in pots. It is best to re-pot them every two years, either into a slightly larger pot, or back into the same pot, after scrapping away some of the soil, and trimming the root-ball a little too. Do this in the dormant season – that would be early spring in colder areas, and fall in warmer ones.

Growing Rhododendrons in Raised Beds

Although growing in pots will be very successful, it does need some work, with re-potting and frequent watering. As well, a collection of pots may not fit well into the style of your garden. If you want to grow more than one or two plants, a more ambitious, but still simple solution is to build a raised bed. Besides making an attractive garden feature, a raised bed for acid-loving plants means you can grow not just rhododendrons, but also blue hydrangeas, camellias, and other acid-loving shrubs and flowers.

Having mentioned hydrangeas, there is something worth adding here. You perhaps already know that hydrangeas produce pink flowers in alkaline soil and blue ones in acid soil. You may have used products to make hydrangeas grow blue flowers in alkaline soil. These usually work well, but don’t be tempted to try them on rhododendrons, thinking it will make them too grow well in alkaline soil. The blueing mechanism for hydrangeas is different from chlorosis in rhododendrons. So that treatment don’t work at all. Which is a pity. . .

Getting back to these raised beds. You can build them out of stone or wood, but avoid limestone, concrete blocks and concrete pavers, or any other lime-carrying materials. Since these plants thrive in the dappled shade of woodlands, near deciduous trees, an appropriate way to build a bed is with logs and branches. This rustic effect fits well into a woodland setting, and allows you to also use up those branches that often accumulate from trimming your trees. Hammer pairs of wooden poles into the ground every few feet, and then build the wall by laying whatever logs and branches you have between them, overlapping them as you need to. You can make these walls in a simple, basic fashion, just make sure they are strong enough to hold the soil behind them. Sloping them inwards a little will make them much stronger.

Why Raised Beds?

It might occur to you that it would be easier to just dig out a shallow area, and fill it with suitable soil. That doesn’t work well, because alkaline water will flow sideways from the surrounding soil, and in a couple of years the bed will stop being effective. By raising the soil up, the natural downwards drainage will stop alkaline water flowing in. Since rhododendrons have shallow roots, the bed only needs to be about 12 inches high. To prevent the existing soil mixing with the soil you are going to add, there are two good strategies. You can line the inside with a layer of plastic or landscape fabric. If you use plastic, punch holes in the bottom for drainage. Another way is to put a layer of coarse sand on the bottom, a couple of inches thick.

Now fill the bed with lime-free soil. For a smaller bed you can use the same potting soil for acid-loving plants that we use in pots. For a larger bed, try your local bulk soil suppliers – they will often have access to acid soil from outside your area. If you use straight soil, add some peat-moss, to improve water-retention and drainage.

Once the bed is filled with soil, you are ready to plant a variety of rhododendrons, azaleas and other acid-loving plants. If the bed is large, add a Japanese maple or two for height and fall color. Those trees thrive in acid soil too. You can make a beautiful bed, with color and interest all year round, and a ‘high-season’ in spring when your rhododendrons will bloom abundantly. In acid soil all these plants will thrive, and grow into beautiful specimens that will be a thrill every year.

Care of Rhododendrons in Raised Beds

The most important care needed is regular water – raised beds drain more quickly than the main part of the garden, and rhododendrons need a steady supply of water. A trickle hose, coiled across the bed, will make watering easy. You can even attach it to a timer, so everything will take care of itself. Some fertilizer for acid-loving plants should be used in spring, and take off the old flower heads when the blossoms fade, to encourage an abundance of flowers the next year. That is about all the attention needed, for the chance to fill your garden with brilliant color, even when your natural soil is not suitable for rhododendrons. This is an easy garden project that will bring huge dividends in beauty and pleasure and take your gardening to the ‘next level’. You will feel like a real pro when you enjoy the results.

Comments 4 comments

  1. June 7, 2020 by Joe and Terry Skafff

    We’ve moved into a house with very mature rhododendrons. Over the past 5 years we have lived here, we’ve had no flowers. Our neighbor told us that they used to flower all the time; absolutely beautiful! I’ve recently found that our soil is very, 8, alkaline. I would like to know what to mix into the soil to treat the akalinity and what fertilizer you would recommend? We would do anything to have flowers next Spring. Thanks, Joe and Terry Skaff

    1. June 8, 2020 by Dave G

      I am confused as to how you can have mature rhododendrons with a pH of 8! I suspect your test didn’t work – you should do several around the area. I suspect that large trees have grown up above them and made it too shady – in which case there is nothing you can do, except have the crowns of the trees thinned, but that won’t be effective for long, as they will thicken up again in a couple of seasons of growth. As for changing the pH of the soil, if it really is that alkaline, the short answer it you can’t. You could try a rhododendron fertilizer, and mulching with acid-free rich material,like well-rotted leaves, but otherwise there is not much to be done.

  2. June 13, 2020 by Debbie Proctor

    I planted my rhododendrons in the South side of my property as a border.i soaked the root ball and covered with fertilizer and added Rodin salt the week after plus rubber mulch. It gets mid day Sun, shade in morning and late afternoon. Since I’ve planted them the blooms died off. Initially, I was watering every other day the 1st week and then twice a day the 2nd week. Green leaves still look good but no more blooms. Is it dead or do I need to just wait until next year for blooms or mice it? I really wanted a border of flowery shrubs for that area. Please advise.

    1. June 13, 2020 by Dave G

      These plants bloom for 2 to 3 weeks, once a year. So perhaps they weren’t the best choice. Rodin (Epsom) salts are not going to do much for rhodos on alkaline soil (if that is what you have) – they provide magnesium, not the iron that they need. Chelated iron is a much better choice. I think you are overwatering, at twice a day – twice a week would be more effective, or perhaps every second or third day for this first season. If you want summer color, why not plant between them with hydrangeas? They will bloom for months, after the rhodos are over. You could alternate them, moving some of the rhodos to other locations when it starts to get crowded – they move easily in late winter and early spring, even when large.