Friday, February 22, 2013

Mountain Mahogany Mulch Misery

I just had a call from a gentleman in Sunland who wanted to know what a good pesticide for Mountain Mahogany (Cercocarpus betuloides) would be. I asked him what the circumstances were. He told me that he had several mountain mahogany plants on his property, but the one located in a partially shaded area had been infested with some kind of white-fly or scale for almost six months. He recounted how he had applied a product containing imidacloprid ( a 'systemic' insecticide that is closely related chemically to nicotine) and that had helped but it wore off and the problem is just as bad. I started asking him some questions:

  • Q. How often is the plant watered? A. Very little supplemental water. 
  • Q. Have you fed the plant?  A. No
  • Q. Have you mulched the plant well and if so, with what?....
                    Now this last question is a somewhat of a trick question....

He answered that he had mulched his plantings with redwood bark. Mulching is usually a good idea -it helps provide organic material to the soil so that microorganisms beneficial to the plant can increase in the soil. However mulching also produces nitrogen, which can be detrimental to many native California plants...especially plants like Cercocarpus betuloides which have roots that fix the plants own nitrogen

So what happens when you mulch a Cercocaprus betuloides with an organic mulch that breaks down to provide nitrogen (a plant whose tissues are chock full of nitrogenous compounds to begin with from its own nitrogen fixing activity)? First you get an increase in green, succulent growth which is attractive to insects like whiteflies, scale, aphids etc. that feed on the food conducting tissue, the phloem, of the plant. To make matters worse besides increasing the plants susceptibility to these insects the increased nitrogen content of the sap caused by the mulch actually helps these pests reproduce! So mulching with a nitrogenous mulch is like pouring gasoline on a brush fire for these plants. Unfortunately for my caller, redwood bark is such an organic mulch (although its nitrogen release is delayed and much slower than other organic mulches), and it can even be toxic to some plants under the right circumstances..so what to use?

 Decomposed granite (D.G.) comes in many different sizes from fine particles that congeal like clay when they dry to almost pebble sized grains. Since D.G. does not break down to provide nitrogen it is ideal for those California natives that fix their own nitrogen and so might suffer if a nitrogenous mulch is applied. So not only is D.G. a good alternative for nitrogen sensitive plants like Cercocarpus but if you're mulching with the smaller, soil-particle sized D.G. you have a situation that can attract native bees which prefer nesting on bare ground. 

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