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Mulching Rhododendrons

Mulch has many benefits when used properly

By: Sandy Feather 2007
Penn State Extension


Q. I have a medium-sized rhododendron whose newer leaves are lighter green than the older growth. I have put coffee grounds onto the soil around it for years, believing that this would help amend the clay soil and make it more acidic. I also paid to have good, rich mulch put over my flowerbeds and shrub plantings this past winter. This spring, the rhododendron does not look good at all (but the weeds are thrilled). Have I overdone the acid treatment? More leaves are going brown, but a few dropping from the largest branches seems normal to me. My neighbors have a huge rhododendron near their house - they do nothing to it, and it looks great! I have never had the soil around my rhododendron tested.

A. There are a number of things that could be going on with your rhododendron. For one thing, it is not unusual for new growth to be lighter green than mature growth. Once the new growth hardens off, and the waxy coating (cuticle) over the leaves becomes thicker, it will become darker green, too. However, leaves that are entirely yellowish or too light green can indicate a nitrogen deficiency.  I would expect the older leaves (those that are lower and more toward the interior of the plant) to be affected more than new ones. And since coffee grounds contain a good amount of nitrogen,  I would not expect that to be the problem.
 


Deficiency Symptoms on Leaves

Leaves that have yellow tissue between green veins can indicate an iron or manganese deficiency. Such symptoms are generally caused by high soil pH, rather than a true deficiency in our area. When soil pH is on the alkaline side, iron and manganese ions remain tightly attached to soil particles and are unavailable to plants that need them. You are correct that it is normal for rhododendrons to lose their oldest, most interior leaves annually.


Test Soil pH

soil test results

Since you have never taken a soil test, it is hard to say if the soil around your rhododendron is acidic enough and provides the proper ratio of nutrients to support optimum growth. Although coffee grounds can have an acidifying effect when applied to soils over a period of time, it is impossible to know if they have lowered the soil pH to the 5.0 to 5.5 range that rhododendrons prefer. Coffee grounds are also relatively high in nitrogen, and should be composted before applying them to your plants.

 


Rhododendrons in Winter

Materials that release nitrogen can force new growth late in the growing season that will not harden off in time for winter. Composting the coffee grounds first should reduce the likelihood of that happening.

Light Purple Rhododendron
Light Purple Rhododendron

Speaking of winter, our short, bitter winter blast was very hard on broadleaved evergreens such as rhododendrons, hollies, English ivy, Pieris and boxwood. In my travels and from samples submitted to our office, I am seeing a lot of winter injury to plants like these. Deciduous trees like maples and oaks drop their leaves, which greatly reduces or eliminates water loss through the winter. Needled evergreens like pines and spruces have a small leaf surface exposed to winter wind, and it is coated with a thick, waxy coating to reduce moisture loss. Broadleaved evergreens have a large, vulnerable surface area of leaves by comparison. They continue to lose moisture through their leaves, via pores known as stomata. Harsh winter winds literally suck the moisture right out of those leaves, but plants cannot absorb additional moisture from frozen soil to compensate for the loss. The result is browning of the tips and edges of exposed leaves; this type of damage can cover the entire leaf when winter injury is severe. They are also more susceptible to winter injury when they are exposed to a lot of winter sun.


VIDEO: Deadheading a Rhododendron
 


Mushroom Compost 'sweetens' the Soil

I am somewhat concerned about the "good, rich mulch" applied over your shrub and flowerbeds in the winter. If it was mushroom compost, it may have actually raised the pH of the soil around your rhododendron. Mushroom compost often has a slightly alkaline pH around 8.0. It is an excellent source of organic matter to improve our heavy clay soil, and provides some nutrients as well. However, it should not be used to amend the soil around acid-loving plants on a regular basis. Also, if the mulch was applied in too thick of a layer, it could be holding too much moisture around the rhododendron's shallow root system and causing a gradual root rot. One or two inches of mulch over the soil surface under shrubs and trees helps keep weeds down, conserves soil moisture, and moderates soil temperature. Mulch should extend out to the ends of the branches (known as the drip line) and never physically contact the stems or trunks.

mushroom compost
Mushroom compost around Spireas

If it was mushroom compost, and it was applied before plants in those beds were completely ready for the winter, it may have interfered with the hardening off process. Shorter days and cooler temperatures get that process in motion: leaf coloration and drop; transport of nutrients produced by photosynthesis to the roots for storage to power next spring's flush of growth; and adjustment of the balance of water and sugars throughout the plant. Hardening off is a gradual process, and our unusually warm winter may have made it difficult for plants to fully harden off before severe cold weather arrived. The combination of unusually warm weather and late season fertilization may have caused more winter injury than you would normally see.

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