Help! Cheap potting soil left every type of lawn weed all over the garden and they won't die.

Asked September 22, 2016, 1:39 PM EDT

A couple of years ago my husband listened to a kid who suggested that he buy a cheaper potting soil than Miracle Grow, and we paid dearly for it. Crab grass is overtaking our lawn and garden. Now we also have creeping oxalis, knotweed, spurge, barnyard grass, foxtail, knotweed, carpetweed and one other that I have yet to identify. My husband tried to use round up one year on the soil after he pulled the plants out. But everything came back. We pull all the weed before planting, but after the crop plants start coming up through the weed cloth, all the weed seeds germinate and take over the garden. I have tired to pull everything before the seeds get going, but I have a medical condition and cannot do much. In addition, we will have the weeds pulled, and then go to the cabin for 3-4 days and when we come back it's as if we never did a thing to the weeds. He doesn't seem concerned that our yard is being overtaken by crabgrass, and the crabgrass is constantly attacking my flower bed as well. I can keep up with that bed only! While we hate, to use Round-up on a food crop garden, is there a smart way to use it safely this fall and not have so many weeds next year? I don't know how else to get rid of the weeds. I realize that one of our biggest problems are the seeds, and they are much too small to see or use a sieve to get them out. Also our graden is too big for that. Please advise us! Thanks for your help>

St. Louis County Minnesota lawns and turf lawn weeds horticulture

1 Response

Weeds can be frustrating for any gardener. Many produce an immense number of seeds which can reside in the soil for many years before germinating. Seeds may be germinated by exposure to light as in when garden soil is turned or tilled; after a cold period (winter); or when the seed coat breaks and water permeates the seed.

Annual weeds like crabgrass, barnyard grass, and foxtail, which grow from seed, can be managed by applying a preemergent herbicide at the appropriate time of year. A preemergent herbicide will prevent seeds from germinating. Note that you cannot apply a preemergent and plant other seeds (grass, vegetable, flowers) at the same time in the same location as they won’t germinate. Annual grassy weeds can also be spot-treated later in the season with herbicide. In preparation for next spring, here is a publication on spring preemergent application for crabgrass.

Broadleaf and grassy perennial weeds in your lawn like dandelion, creeping charlie, oxalis, knotweed etc. can be managed with an herbicide blend of glyphosate (the active ingredient in Round-Up) and triclopyr. (I believe Weed-B-Gone is one of the brands, but there are a variety of products that contain these chemicals). Note that these chemicals will kill other plants, so be careful to protect desirable plants such as ornamental perennials and annuals from the chemical including from drift (being blown by wind). Again, read the labels on the chemicals at the store to choose the right one for the weeds you are trying to kill.

Here our Extension webpage “Is this plant a weed?” that contains specific information on weeds.

IMPORTANT: It is important to choose the right product for the situation. Make sure the plant or type of plant you want to treat is listed on the label. If you are treating near edible plants like in a vegetable garden, be sure the product is designed for weed control around edibles. Apply the chemical at the proper rate and frequency as well as time of year. Always wear the proper protection and all products should also be stored as directed on the label. Follow the label - the label is the law. An example of improper application is spraying Round-Up on the soil. Round-Up must be sprayed on the plant itself to be effective. Glyphosate is absorbed into the plant via contact with the leaves, stems, flowers, etc. and transferred to the roots of the plants.

Garden bed: To minimize weeds in your garden bed, treat with treat the areas glyphosate / triclopyr combination kill any remaining perennial weeds. Be sure to protect any perennials (flowers, asparagus, etc) from the spray. Remove all plant material after it has died back. You could cover the ground with weed barrier to prevent any additional seeds, etc. from blowing onto the soil surface.

In the spring (April), apply a preemergent and a slow release fertilizer to the soil surface. Water it in according to the instructions and put down your weed barrier. Be sure the weed barrier is of a permeable material that will allow water to flow down through it to the soil, but prevent weeds from growing up through it. Do not use black plastic. Use drip irrigation or soaker hose emitters to water your plants. Position the hoses at the base of your plants so the water is delivered right to the root zone. The combination of the preemergent and weed barrier should prevent most of the weed seeds from germinating. I recommend planting vegetable and annual flower transplants, not seeds, while you are getting the weed seeds under control. When you plant the transplants, cut a hole through the weed barrier that is not too large. The weed barrier, preemergent, fertilizer and direct irrigation will really minimize plant disease in your garden as well as warming up the soil and reducing weed pressure. Dig / pull / spot treat any weeds that emerge.

Lawn: Maintaining a healthy lawn will help reduce weeds. Choose the correct type of grass is a good place to start. Fescues are good options for shady or sunny areas and tolerate drought better than our traditional Kentucky bluegrasses. Raising your mower height to about 3” will keep grass blades longer and reduce the effects of heat stress. Fall is a good time to reduce soil compaction (common in home lawns) through core aerification (removing soil plugs from your lawn to increase air space and improve root growth). Follow by spreading grass seed, fertilizer and a good quality compost (without weed seeds) and give your lawn a god headstart in the spring. Here are some resources including our lawn care calendar that will help you determine the best time of year to apply weed control as well as perform other lawn care functions:

Feel free to reply to this email with further questions.