Ask a Master Gardener for June 15, 2014
Q: I have been seeing lots of posts online about an organic way to kill weeds. It is vinegar, Epsom salts and dish soap. Does it work?
A: The vinegar will kill the tops of weeds. Repeated spraying will eventually weaken the plants and may kill some young or weak weeds, but it usually does not kill mature or tough ones. Vinegar is acetic acid and can cause environmental damage. This “organic” mixture is more toxic and less effective than glyphosate (Round-up is one brand), which works by killing the roots and is very low on the environmental impact quotient scale. If you do not want to use any chemicals, you are best off hand-pulling the weeds and using mulch, not chemicals from your kitchen. Q: I have a pagoda dogwood in full sun that contracted golden canker. I had been pruning the dead branches and cut the tree down a few weeks ago, but left the new growth coming from the base. Is it possible for this tree to have “walled off” the canker disease and that this new growth can flourish? A: Golden canker is a common fungal disease of pagoda dogwoods, even those grown under ideal conditions. It does not affect other kinds of dogwoods. Infected branches turn yellow or tan and may become covered with small orange bumps. It is easiest to see the signs during the winter, but if a branch fails to leaf out or the leaves subsequently die, look for the yellow color. The problem usually starts at a branch tip and can progress to the trunk, but the fungus does not kill the roots. There is no fungicide or other chemical that will eliminate the problem. A study showed that over 60% of branches that appeared to be healthy were actually carrying the fungus, so it is hard to know if your new shoots are already infected or if they eventually will be. At this point, if you would like to keep your tree, I would recommend you watch it carefully. At the first sign of a problem cut the infected branch off right above a bud that is about 4-6” below the discolored bark. Make sure to sterilize your shears between cuts (30 seconds in a 10% bleach solution works well) and dispose of the branches and the fungus spores by removing them from your property. If there are orange bumps present be especially careful as they are the fruiting bodies of the fungus and you don’t want them escaping. Pagoda dogwoods are naturally understory trees, meaning they grow in the cool, moist shade of larger trees. Full sun stresses them and increases the severity of canker damage. If you cannot move it to a shadier area, you should keep the roots cool by adding 2” of organic mulch around the base of the tree and make sure it gets plenty of water. Q: How thick should the layer of gravel for drainage be in the bottom of my houseplants or outdoor pots? Does it depend on the size of the pot? A: Actually, the thickness should be the same in all pots — zero. We have been told for years that this layer of gravel helps with drainage, but many scientific studies show the opposite. Water doesn’t travel well from one type of medium to another. When it starts with potting soil and hits gravel, rocks or even sand, a lot of it winds up just sitting above the gravel in something called a perched water table. Of course some water does eventually drain through the bottom drainage hole (this drainage hole is very important in all pots), but some stays at the bottom of the soil. You are better off filling the pot completely with potting soil. If you have a really big decorative outdoor pot, you can fill the bottom with something to avoid having to use a lot of potting soil, but make sure to allow plenty of space for soil. You can put a piece of screen, a coffee filter, or something similar over the drainage hole to keep the dirt from washing out, just make sure that it doesn’t plug the hole.