What do you do if you want to kill grass under a mature tree so you can put ground cover in a big circle around the tree? Can one safely use black plastic that will deny water to the tree? Maybe use weed block? How long do you leave on whatever cover you use to kill the grass?
Hardiness Zone: 4a
By Bill C. from Minneapolis, MN
You can use a weed killer spray and the next day it's usually brown. The grass will grow back eventually, but if you have the area worked up with a tiller or other tools after spraying and then plant your ground cover, it shouldn't over take the area.
May family calls me the "cardboard bandit". I collect heavy cardboard from the neighbors recycling bins and use it to make landscape beds. It works very well. Use a rope or old hose to mark the edge of your bed. Dig an edge for the bed, about 4" wide. Lay cardboard over the entire area, bending it about 4" at the edge, going into the area you have made for an edge. Make sure you overlap the cardboard so no light gets through. Cover the cardboard with compost and 3" of mulch. Wood chips / bark are best. I have used this method for 10 years. Every year add more mulch. Newspaper also works, but it must be lots of layers. The cardboard and mulch help the soil. My older beds are like peat moss with mulch on top. Almost no weeds, even after years.
Oh Please, do not use weedkiller. It kills living things, including your tree. The grass that dies under the cardboard method, become green manure.
If you wet the cardboard in a big plastic tub it is easier to place. It will conform to the ground and not blow away before you get mulch on it. I peel all the tape off boxes before I use them, as it does not decompose well. My tomatoes pulled through a drought this summer with cardboard, a couple inches of straw, and one supplemental watering! I don't bother with the trench when I begin, maybe later when I decide to put a permanent edge in a the soil has softened from the mulch.
In a couple years, when your ground cover covers things completely, you will probably not want to add much mulch. It will be too difficult to get it in there!
Do not till under trees, they usually have a lot of roots near enough to the surface that you can damage the tree. Also, those root are there in part for oxygen, so don't bury them too deeply with mulch, especially don't decide to make a huge raised bed and dump tons of topsoil there. The tree may not die immediately, but will decline.
Jadeibert, if you're going to be a tree hugger, go all the way. Cardboard commonly contains formaldehyde, a very nasty chemical. I'll take Roundup any day and it would not kill the tree. It's systemic and only travels through the root system of the grass being sprayed and is supposedly deactivated on contact with soil.
You can spray vinegar directly onto the grass, being careful not to get it on anything you don't want to kill, and avoid if possible getting on tree roots. They do not like it! This will turn the grass brown, and then you can get it up; however, if you want to, you can easily use a "twister" which loosens the soil, and makes it easy to remove the grass with the roots; shake the soil off of it, and then you can put whatever ground cover you wish under it. If you choose creeping thyme it will cover over and squeeze out any weeds and is lovely for the nose to smell!
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