Thursday, July 03, 2008
It's too late to cut back rhododendrons
John Arbogast
Landscape consultant John Arbogast answers your questions every Thursday. Send questions about your lawn, garden, plants, or insects to:
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5102 Greenfield St. SW
Roanoke, Va. 24018
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Q: I live in Southwest Virginia, and my house is surrounded by 10-foot-tall rhododendrons that are covering my house. I need to cut them back but it is early July and I am wondering if I should do this now or wait until spring? If I cut them back one-third at a time, when will I see new leaves and flowers? Also, if I have to remove them, how wide is the root system with plants of this size? I ask this because my house is over 100 years old and I do not want to damage the foundation of our old house. Do you have any suggestions? Please help!
A: With the lack of precipitation that much of this region of Virginia has received during the past 18 months, I would not cut back naturally stressed rhododendrons this late into the growing season. Cutting off the end of a living branch sends a signal to the vegetative buds growing on those branches just below the cut that they must produce new shoots soon to begin to replace the lost parts. So, even though cutting your rhododendrons back one-third at a time would attempt to meet your objective, the plant energies needed for rejuvenation would be lacking under midsummer stressful environmental conditions. Waiting until next early to mid spring should stimulate immediate recovery under better growing conditions. “Rejuvenation” is the horticultural term used for moderate to heavy cutting back of leafy plants in the spring for the purpose of turning them into new smaller plants.
You’ll want to see new rhododendron leaves begin to emerge within two weeks after cutting. Thorough watering of cut-back rhododendrons once per week during dry times of the whole growing season following cutting will be helpful if allowed.
If your rhododendrons bloom late in the spring, early spring rejuvenation will remove flower buds before they can open. So, you will not have flowers for another whole year. Fertilization will not help.
My opinion is that you won’t have to remove the rhododendrons for fear of their damage to the foundation of your old house. One reason is that over the past 100 years, some lime has likely leached out of the concrete or mortar of the house’s foundation making the soil closer to the house less acidic there than the soil further away. Since rhododendrons need acidic soil, my guess is that you’ve had less rhododendron root growth close to the foundation with most of the roots growing into the soil away from the house.
The roots of your 10-foot-tall rhododendrons most likely extend out below ground as far as the branches hang out above ground. Soil quality and moisture are important factors, though, since rhododendrons will follow the easiest path available for root growth. The roots of this plant are not aggressive.
Since you should not cut back your 10-foot-tall rhododendrons now, my only suggestion for this time would be to draw attention away from the overwhelming rhododendrons by planting a border of colorful annual flowers in front of the rhododendrons now if you can still find annuals. I’m guessing that your house is in the shade, since rhododendrons need shade. So, I’d suggest planting impatiens, shade tolerant begonias, or sweet potato vines with fluorescent yellowy-green leaves. You’ll have to water the annuals two or three times per dry week.
Q: A friend’s tomato plants are growing tall and lush with some big green tomatoes here and there. I asked them what their secret was, and they wouldn’t say. They did say that they didn’t use any special garden fertilizer before planting, because they were afraid of blossom end rot. They do use some colored plastic mulch between the tomato rows and grass clipping mulch in other parts of their garden. What do you think is their secret?
A: The colorful plastic mulch between tomato rows might help, because it is shiny and does reflect sun rays back up to the tomato leaves. I couldn’t find any information on the increased value to tomato photosynthesis of certain colors of plastic over traditional black plastic. It is known that certain colors of the light spectrum are needed by growing plants as opposed to colors of the light spectrum that we humans need to see and read.
Faithful regular watering every few hot days is definitely helpful for lush tomato growth. This is why tomato and other garden transplants are planted with soil bowl shapes in which the garden soil level in a 8 to 10 inch radius out from each transplant trunk is lower than the garden soil level around the hole to hold water that can slowly seep down to the roots instead of running off to other parts of the garden.
We can’t forget the importance of good soil quality with lots of organic matter to encourage vigorous tomato root growth. This could explain some of your friend’s successes.
