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Mulching Rhododendrons |
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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. 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.
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. |
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