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Wednesday - May 25, 2005

From: Fort Collins, CO
Region: Rocky Mountain
Topic: Diseases and Disorders, Pruning, Transplants, Shrubs
Title: Smarty Plants on mockoranges
Answered by: Joe Marcus


I live in Colorado where it is common to have 1 or 2 late frosts. I planted 4 littleleaf mockoranges (Philadelphus Microphyllus) 2 years ago and they are pretty much in full sun most of the day. I have all 4 on the drip system and water twice per week (each have 2 emitters and drip about 40 minutes each time). I have redwood mulch around each of them to a depth of about 3 inches. I fertilize them once in the early Spring and once again in the Fall and I never prune them. One of the bushes is growing beautifully with many leaves appearing this Spring. The other 3 bushes have very few leaves and by July look like dried-up tumbleweeds. I realize they are probably trying to get established but I am very frustrated because I feel maybe I'm doing something wrong. I'm learning that their leaves appear on wood from the previous season so am frightened to prune off any of what appear to be dead wood. I am tempted to remove the 3 and plant new littleleaf mockoranges in their place and just start over. Could you offer advice how to get them healthy and full of leaves? I have searched the internet but there is little or no information available on them.


It sounds like the problem with your mockoranges has to do with the roots. The most likely scenario is that the three problem plants were pot-bound (root-bound is another term for the same phenomenon) when you purchased them and they have never grown out of that condition. Pot-bound plants will often continue to grow spiraling roots long after being planted into the ground and may never grow normal roots. In these case, the plant continues to act as if it's still in a pot and will suffer during extreme weather such as that which you experience in the summer. When transplanting a pot-bound plant, you should prune away the roots that are circling the bottom of the pot. Also, loosen the rootball and physically straighten some of the larger roots to re-orient them to a more natural, radiating direction. Of course, anytime you remove roots you should also remove some of the top of the plant. For severely pot-bound plants from which you remove a large portion of the roots, you might have to remove 1/2 or more of the top of the plant. When you do this, try to remove older wood and shape the plant if possible to encourage healthier growth. Sometimes, plant roots can grow around one another and actually strangle themselves. This is most commonly seen with pot-bound plants, also. It is possible that the hole in which you planted you shrubs is acting like a pot and is not allowing the roots to penetrate into the surrounding soil. This is most common in clayey soils. Finally, the three problem plants may be suffering from a root-borne disease that has not infected you one healthy plant. Chances are, the disease would have already been in the rootball of the plants when you bought them. However, it could also have been in the soil where they were planted, especially if they replaced other plants that had died there previously. One last possibility is that your irrigation emitters are not watering properly. That is easy enough to check if you suspect that that might be the problem. Simply run the irrigation into some containers of the same size and compare the amount collected. If you do that, be sure and return the emitters to the same location where they were before. Plant become acclimated to irrigation drip emitters and develop their roots in response the the water zone that develops there. If you move an emitter, you can actually cause water-stress problems by drying out the area where most of the plants' fine, water absorbing roots are located. I would suggest digging up one of the problem plants and examining the roots. If the roots look much like they did when you planted the shrub, then it is likely pot-bound. In that case, try the pruning technique I mentioned earlier and replant. If you see lots of dead roots, there may be a fungal disease involved.


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