Weed Prevention Tips & 6 Common Mistakes to Avoid

Think it’s an overstatement to call it the war against weeds? Here’s what you’re up against.

A single redroot pigweed is able to produce up to 30,000 seeds in a season. And those seeds can remain alive in the soil for 70 years waiting to sprout and overrun your perennial border at any time.

Controlling weeds is a fight you can’t win entirely because they always grow back. But you can keep weeds under control by depriving new ones of the conditions they need to take root in the first place. Let’s look at how to prevent weeds from growing.

(For those of you who already have weeds attacking your yard, read our article on How to Get Rid of Weeds.)

Weed Prevention

As with most types of prevention, discouraging weed seeds from sprouting requires some extra time now so you can save a lot of time later.

Spread Landscape fabric and cut it to fit around plants. | Photo by Saxon Holt

Fertilize Enough, but Not Too Much

Too little fertilizer can lead to sparse lawn that loses the competition with weeds. Too much helps nurture certain weeds, notably annual bluegrass, Bermuda grass and crabgrass. Strike a balance by following the application rates on the package. And use a fertilizer with a high percentage of controlled-release nitrogen, such as sulfur-coated urea, ureaform or IBDU. These provide a slow, steady nutrient supply.

The frequency and timing of your fertilizing efforts are also crucial to healthy lawns. Both vary depending on your lawn type and the length of your growing season. Most northern lawns need only one or two applications of fertilizer annually—once in fall and sometimes a second time in spring. Southern grasses might require three feedings—early to mid-spring just after the grass greens up, early summer and again in early fall.

Water Grass Infrequently and Deeply

Frequent, light watering causes shallow roots and helps annual bluegrass, crabgrass, chickweed, sedges and other weed seeds germinate. If you water too little, the lawn suffers while spotted spurge, Bermuda grass, quackgrass and other weeds adapted to drier soil thrive. Instead, provide your lawn with infrequent, deep soakings. Lawns need about 1 inch of water per week. Set an empty tuna can on the lawn to determine when you have applied 1 inch of water.

Can I Put Landscape Fabric Over Weeds?

Yes, you can. Synthetic landscape fabrics provide a physical barrier to weeds yet allow air, water and nutrients through to plant roots. Spread the fabric over bare soil around trees and shrubs; overlap several inches of fabric at the seams. Anchor the material with U-shaped metal pins, then conceal it with 1 to 2 in. of mulch, such as stone or bark chips.

You can also use landscape fabrics to control weeds under decks and in pathways (spread over the excavated soil base before you add gravel or sand). A 3×50-ft. roll of landscape fabric, such as the Typar shown below, costs about $10. The fabric is also available in 36-in. die-cut circles (about $3 each) for installing at the base of trees.

Smother Weeds with Mulch

Left unattended, weeds will quickly fill in unplanted areas and any open ground around plants. Mulch spread over the soil surface blocks the sunlight most annual weeds need to take hold. Weeds that do sprout are easy to pull because soil beneath mulch remains loose and moist. Coarse chipped or shredded bark is a good choice for large areas between trees and shrubs because it decomposes slowly and doesn’t easily blow away. For paths, a thick layer of sawdust provides good weed suppression because it depletes nitrogen in the soil.

How to Mulch Over Weeds

After clearing a landscaped area of visible weeds, put down coarse-textured mulch up to 4 in. deep.
Apply a fine-textured mulch that packs tightly, such as shredded leaves, to a depth no greater than 2 to 3 in.
Keep the mulch a few inches away from the trunks and stems of plants to prevent disease problems.

Apply Preemergence Herbicides

Preemergence herbicides, such as those containing oryzalin or trifluralin (look on the label for these chemicals), or nontoxic corn gluten meal, kill weeds just as they germinate and will not eradicate established weeds. For a preemergence herbicide to be effective, you must apply it to soil cleared of visible weeds; also, you have to water most of these herbicides into the soil.

Check the label to determine if it is safe for use around the kinds of landscape plants you have and effective against the weeds normally present.

Deprive Weeds of Water

Weeds can’t survive without moisture. In areas with little or no summer rain, drip irrigation or soaker hoses help prevent weed seeds from sprouting by depriving them of water. These systems deliver water to the root zone of plants at the soil level. The soil surface and area surrounding the plants stays relatively dry. In contrast, overhead sprinkler systems spray water over the entire soil surface and supply both garden plants and weeds with water.

You can get in-depth information on drip irrigation from the Irrigation and Green Industry Network in the “Where to Find It” section.

Mow Higher

Mowing too low weakens turf by reducing the ability of a grass leaf to produce enough nutrients. It also lets light hit the soil surface, which helps crabgrass and goosegrass seeds sprout and grow. Check with your local extension service for the recommended range of mowing heights for your grass type. Then mow at the highest level—usually between 2 and 4 inches.

Any weeds that grow through mulch are easy to pull because the soil remains loose. | Photo by Saxon Holt

6 Weeding Mistakes

In the process of trying to eliminate weeds, people often make mistakes that lead to more weeds. Here are the most common:

Leaving weeds that are in flower on the ground. Even after they are pulled, weeds like chickweed and purslane can continue to develop seeds.
Piling too much mulch over landscape fabric. As the mulch breaks down, it provides a perfect medium for weed growth from wind-borne seeds. You can actually have weeds rooted to the fabric. Limit mulch depth to 1 or 2 in. over landscape fabric.
Applying mulch containing weed seeds. Sometimes mulches such as straw and wood chips contain weed seeds. To avoid this problem, buy from a reputable nursery that offers mulch free of weed seeds.
Tossing weeds with seeds into the compost pile. A good compost pile can get hot enough (160°F) to kill weed seeds. But there are often cool spots where the seeds can survive. Those that do will be spread in your garden with the compost.
Breaking apart the roots of perennial weeds as you try and dig them out. Each piece can grow into a new plant.
Planting weeds along with your new shrubs and trees. Just a few nutsedge or Bermuda grass plants growing in a nursery container can spread and multiply in your garden. Make sure to remove them before planting.