Digging In - Advice on Foot-Tolerant Plants, Dying Azaleas
Thursday, January 8, 2009
Q You occasionally advise readers on plants that will take foot traffic and can be substituted for lawn grass. Have you heard of a line of plants called Stepables?
A Stepables are part of a marketing program of about 150 plants chosen and tested to tolerate the effects of feet. Many, such as chamomile and thyme, have been used in gardens for centuries. Others, such as creeping wire vine, are new to garden centers. I applaud the effort to introduce consumers to alternatives to turf. In my garden, I have a path that is a rich tapestry of many of the plants marketed as Stepables by the Under a Foot Plant Co.
The company is forthright about how much traffic each type of plant can take, reporting, for instance, that Elfin thyme can put up with human feet three or more times a day and that the Sternkissen dianthus will take only occasional foot traffic.
Our once-extensive planting of 50-year-old azaleas has been dying out in recent years, and replacement azaleas are dying, too. Can the soil have gone bad? We did have ivy nearby. Could that have harmed the azaleas?
English ivy has been reported to exude chemicals that can prevent growth of other plants, but its effects should not last for more than a year or two after you remove it. A change in drainage pattern, poor-quality mulch or mulch that is too thick might explain the symptoms you are seeing.
In addition, the drought in 2007 and last year's deluges have been difficult for azaleas and rhododendrons, both of which have most of their roots in the top six inches of soil. They dislike the wild swings in soil moisture that we have been experiencing.
And with new azaleas, it is important to break up the root system when you plant them so the roots don't maintain their pot shape in the ground. You should also make sure not to plant the shrub deeper than it grew in the pot. Mulch should be kept several inches away from the crown of each plant.
It would be a good idea to test your soil's pH. Litmus paper and most meters available for less than $100 aren't very accurate. I once tested a two-prong meter I bought in a garden center and found that it rated the pH of lemon juice, tap water and chlorine bleach all the same, at pH 6.8. Invest in a reliable soil test from a testing lab (find numerous vendors by searching the Internet for "soil testing lab") so you know exactly what the pH is. The wrong kind of mulch used in anaerobic conditions can drive pH up to levels that are deadly to azaleas.
If a soil test reveals that you need to reduce the pH, use iron sulfate or aluminum sulfate according to label directions. Elemental sulfur can be used to reduce soil pH, but microbial action is also needed. It works best when the starting pH is 6.3 or lower.
Scott Aker is a horticulturist at the U.S. National Arboretum.