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Monday - September 21, 2009

From: Williamsburg, VA
Region: Mid-Atlantic
Topic: Invasive Plants
Title: Controlling vegetation around retention pond in Williamsburg, VA
Answered by: Barbara Medford


We planted Juncus effusus around a retention pond and various native shrubs last year. We are having a problem controlling bramble,lespedeza and broadleaf natives from taking over and native trees(willows, maples, tulip trees) from growing up everywhere. We need advice on groundcovers and how to control everything without using toxic chemicals if possible. Homes back up to the pond, so we want to maintain the water view without destroying a natural buffer that filters pollutants and provides a habitat for birds and small animals.


If you will excuse the use of technical terms, you have a real mess there. It sounds like you have made a good start in the plants around the retention pond itself, but before you even consider planting anything else, you need to get a whole lot of other stuff out of there. You are correct that the use of wide spectrum herbicides would be a poor choice. You can't teach an herbicide to kill only the plant you don't like. They can not only kill everything they can get at, but they can leach into the soil and excess can be washed on downstream into rivers or lakes and ultimately your drinking water. 

To deal with the plants you have now, "bramble" can refer to several different plants, including members of the genus rubus, in the rose family; for instance, Rubus lasiococcus (roughfruit berry). Other members of the genus may be called blackberries, raspberries or thimbleberries. All share the trait of long arching limbs, usually with thorns, that can engulf an area if not controlled. In a simlar vein are some members of the pea family, like Mimosa microphylla (littleleaf sensitive-briar), which also has long, skinny branches with prickles all over them. They have deep taproots and must be pulled out very thoroughly, or they will just come back up again. Unfortunately, where you are going to have to start is probably grubbing out all the brambles, by whatever name, that are out there, because they will obscure what else you need to do, and certainly impede you from doing it. If some of them have very large roots, you can cut them off as near to the ground as possible. Then, in five minutes or less. use a disposable paintbrush to coat the cut surface of the root. Doing this quickly, before the root heals over, will help to get the herbicide down to the root. Be very careful while doing this, again, to avoid damage to other plants you wish to preserve, or going into the soil. 

Lespedeza sericea, a perennial, is another legume or member of the pea family, and is native to Japan. It is often used for grazing purposes or for cover crops.  If the plant you have is an annual, Kummerowia stipulactea, also from Japan, it can be controlled by mowing or removing the plants before they have the opportunity to produce seeds.

Now, on to the trees you have. Please see this previous Mr. Smarty Plants answer to see how we feel about willows, especially weeping willows. While the Salix x sepulcralis or Salix x babylonica, weeping willows, are non-native to North America, there are 55 members of the genus salix that are native, of which 10 species are native to Virginia. Even if your willows are natives, you will find them fast growing, somewhat invasive and often pest and disease-ridden. Keeping them is your choice, but we wouldn't recommend planting any new ones. As for the other two trees that you mentioned, Liriodendron tulipifera (tuliptree) is native to Virginia; there are 13 species of the genus Acer native to Virginia, perhaps Acer rubrum (red maple). These are both very valuable, attractive shade trees, but they both get BIG. If you have them popping up all around, you need to select the two or three trees you wish to keep and maintain to grow to large shade trees, and be vigilant about pulling out sprouts, and cutting down and using the paintbrush method explained above to keep the saplings under control. 

Now, your basic question was for advice on controlling everything and groundcovers. First things first, as we said before, you can't put down any groundcover and expect it to flourish until you have taken out all the invasives. We are hoping this retention pond is a project of a homeowner's association, and that there will be help forthcoming from that association. No matter how much we would wish it, there is no plant, or at least not one you really want, that could be put in there that would control the unwanted plants. Our suggestion is, now that's it's Fall and the weather is more pleasant, that the troops or a contractor or whatever is possible be rallied and the clearing and hauling away be started right away. Since Williamsburg, on the Virginia Peninsula and bordering on the Atlantic Ocean, is likely USDA Hardiness Zone  7b, that means you have a minimum average annual temperature of 5 to 10 deg. F. That is not going to be cold enough to retard regrowth of the invasives you have (hopefully) taken out, but should slow them down. Then is the time to start thinking of groundcovers to put in. Rather than give you any suggestions on that now, when we don't know what the sun exposure is going to be, how moist the soil is and so forth, why don't you get back to us in early Spring, maybe February, with clearer specifications and indications of what has been accomplished in the clearing. We can make more appropriate suggestions at that point. 

Acer rubrum

Liriodendron tulipifera

Rubus argutus

Mimosa microphylla





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