It saves a lot of trouble to get rid of the weeds in a garden before you plant it. This may be obvious if you are planting groundcover in the summer on a previously—unplanted bank that’s covered with weeds. But it may also be true when you landscape a new home whose yard is all bare soil, or even when you take up some sod to make an vegetable garden.
Weeds exist in different forms. The obvious ones are live, growing plants you can see. But weeds might also exist in underground dormant plant parts that you can’t see, such as roots or tubers or bulbs. And they always are present as seeds hidden in the soil.
Some weed seeds sprout as soon as conditions are right, but others can remain dormant for years, sleeping until you turn over the soil and expose them to the oxygen and light near the surface.
1. Kill the Growing Weeds
The basic principle of weed control in new land is to encourage the weeds to grow, then kill them. The only way to kill dormant weeds or seeds is to fumigate the soil, a process that must be carried out by professionals, and which kills all life in the soil. But once they are actively growing, they are susceptible to control.
In spring or summer, when the weeds are actively growing, kill the weeds with weed killer. Herbicides kills all the weeds, roots and all, then breaks down in the soil so it doesn’t interfere with the growth of garden plants.
If weeds are dormant, in early spring or fall, wait until they are actively growing. Although it’s tempting to clean off the dead weeds and plant, you might be dismayed to see what sprouts the following spring.
Some weeds can be successfully pulled by hand, scraped off with a hoe, or killed with a contact herbicide like weed oil. However, this is a waste of time with many persistent weeds that will only resprout from parts left alive in the ground. If you are sure you don’t have any persistent weeds, you can skip this step.
2. Remove the Weeds
A week after spraying with Roundup, clean off the weeds with a hoe. They may still be mostly green, but their underground parts have been killed and they will not re-sprout. By this time, the Roundup has broken down and is no longer active in the soil.
3. Germinate Weeds Seeds
When the growing weeds are gone, spread any amendments and turn over the land. See Preparing Soil for Planting for information. Shape the soil the way you want it, create paths and beds and smooth off the surface. Then water it well and keep it moist for a couple of weeks to germinate weed seeds. Spray these seedlings with Roundup and wait another week. If no more weeds sprout, you are ready to plant.
Once you have germinated and killed the seeds near the surface, avoid turning the soil over again. This will only bring more buried seeds to the surface to germinate.
If you have the time and want to be very sure the weeds are gone, wait for the next growing season to plant. Different weed seeds germinate under different conditions. Although most germinate with moisture and warm weather, some germinate in cool fall weather and others need a period of cold to break dormancy, then they germinate when the weather warms up in the spring. Keep the soil moist and kill any germinating weeds before they go to seed (or you will have to do it all over again). You might be surprised to find a sudden dense crop of chickweed or annual bluegrass germinating in September.
Other Ways to Kill Weeds
Weeds can be killed or kept from germinating by keeping them in the dark. All plants need light and starve without it. There are two ways to use darkness to kill weeds.
One way is to cover the ground with an opaque cover for a long time before planting. While the soil is moist, spread black plastic or weedblock fabric over the ground, weeds and all, and weight down the edges with soil or rocks. Keep the soil moist to sprout weed seeds. Within a month, the weeds under the fabric will be dead. If you leave it in place for a year, all the seeds that germinated during that period will also be dead and the soil will be bare.
This method doesn’t work well if the weeds are established bermudagrass or quackgrass. (Quackgrass is also called couchgrass, dog’s grass, twitch, Agropyron repens and some other names that shouldn’t be printed.) These plants have such extensive rhizome (underground stem) systems that uncovered plants yards away will nourish the part of the plant covered by plastic. An opaque cover can be used to kill these grasses, but all the grass must be covered.
To avoid stirring up more weed seeds, don’t till the soil before you plant. Set out plants rather than seeds and cover the soil with a mulch. Improve the soil by adding amendments in each planting hole, then pull the mulch back around the stem of the plant. This method works well for ground covers or perennial plants that can be set out as growing plants and mulched.
Another light-excluding method is to cover the weedy ground with organic matter that decomposes. Use compost, manure, straw, hay, leaf mold, or any other material that will decompose within a growing season. Spread it at least 4 inches deep, or deeper if you have vigorous weeds to kill. If you wish, you can spread a few inches of soil on top of the organic matter and plant annual vegetables or flowers in it. The annuals will grow into and through the organic matter.
This no-till method establishes gardens quickly without disturbing the soil, thus keeping more weeds seeds from being brought to the surface and germinating. Earthworms and insects carry the organic matter into the soil, improving it and “tilling” it for you, however slowly.