How to Kill Weed Seeds in Soil [5 Easy Methods]
To kill weed seeds in soil you will have to apply one or more of the following methods:
- Heat soil to temperatures high enough to kill weeds seeds
- Force seeds to sprout and destroy growing weeds
- Apply chemical or natural weed killers that prevent weeds from sprouting
- Use flame weeding to destroy weeds and seeds at once
- Layer mulch in garden areas to suppress weed sprouting and attract insects that eat seeds
With this arsenal of tricks for killing weed seeds before they sprout, you can stop the spread of weeds in both your lawn and garden.
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5 Tips for Killing Weed Seeds
Rather than spending time and effort battling weeds as they sprout, attack weeds at the source by killing weed seeds. Each of these methods destroys weed seeds, which ensures you won’t have to battle recurring weed invasions. Try these ways to get rid of weed seeds yourself.
Soil solarization is a very powerful method for killing weed seeds. Weed seeds begin to die if soil temperatures surpass 108 degrees, with full seed death ensured by soil surface temperatures of 140 degrees or more. Solarization uses clear plastic tarps to trap heat at the soil surface, killing weed seeds within the tarped area. To solarize an area, follow these steps:
- Clear the area of all vegetation through use of a hoe or other garden implement. Remove any woody stumps
- Till the soil to further break up any weed root systems left behind.
- Rake away all vegetation residue
- Water the tilled and cleared soil with a garden hose until it is damp.
- Lay a sheet of clear plastic over the area. Weigh it down tightly at the edges
- Leave the plastic in place for at least two months.
Solarization is the best method to reclaim a weedy garden or other area. It is a “clean slate” for your soil, because seeds will be destroyed by the solar heat trapped beneath the plastic.
It is typically tough to implement solarization in large areas and is not usually suitable for use in lawns, where you may want to preserve grass or other plants. Pre-emergent weed killers and flame weeding are much better for use in lawns.
Till and Kill
Weed seeds can lie dormant in soil for decades and are only “activated” when brought to within an inch of the surface. One method to rid soil of dormant weed seeds is to force these dormant seeds to sprout, then attack them with a powerful natural or chemical weed killer. To do this:
- In spring, till the affected area. Tilling brings dormant seeds to the surface
- Water the area for 1–2 weeks with a sprinkler or soaker hose
- When weeds begin to sprout, apply the weed killer of your choice
This is another “clean slate” method, where you force weed seeds to show themselves and then kill young weeds before they mature and cast seeds. Because of the invasive tilling step, it is not best used in areas with desirable grasses and plants.
Use Pre-Emergent Weed Killer
Pre-emergent weed killer stops weeds in their tracks. It works by attacking weed seeds just as they begin to germinate, killing them before they even poke above the surface. It’s a weed killer so good, the only sign it’s working is that there will be no new weeds at all.
How to Prevent Weeds From Growing
Tips on how to keep weeds out of the garden, add the right amount of mulch over weeds, and 6 mistakes to avoid to keep your garden weed-free.
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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.)
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.
Photo by Saxon Holt
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.
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.
This Preemergence herbicide, made from corn gluten, is nontoxic. You can safely use it near all of your vegetables as well as around ornamental plants. Photo by Saxon Holt
Where to Find It
Lee Valley Tools Ltd.
Ogdensburg, NY 13669-6780
Telescoping Crack Weeder
True Temper Hardware
Camp Hill, PA 17011
Drip irrigation information and supplies:
Irrigation & Green Industry Network
916C N. Formosa Ave.
Los Angeles, CA 90046
2250 Agate Ct.
Simi Valley, CA 93065
Request the free “Drip Watering Made Easy” guide.
Denman & Co.
401 W. Chapman Ave.
Orange, CA 92866
Cherry Valley, IL 61016
Top Strategies For How To Get Rid Of Weeds
Eradicating weeds isn’t a task for the fainthearted. It requires elbow grease, persistence and a little intelligence. In order to know your best method to kill any particular weed, you need to understand it — how it grows, when it grows and the best time to attack it. Improve your odds of success by adopting some of these clever, and easy, weed-killing strategies.
From the moment annual weeds sprout, they’re racing to flower and set seed. Your job is to interrupt the process. If you can’t kill weeds or pull them shortly after they appear, make sure you deal with them before they set seed.
If they set seed, mowing with a grass-catcher bag attached is a way to cut and collect seeds before they drop. This method works well with grassy lawn weeds, such as Quack Grass and Goosegrass. Rake weeds before mowing to pull seed stems upright. After mowing, destroy or dispose of seeds – don’t add to your compost pile. Be careful not to rake if seeds are dropping, or you risk spreading seeds.
As soon as perennial weeds sprout, it’s time to take action. With tap-rooted weeds such as Dandelion, pulling young plants improves your odds of removing the entire root.
If you miss this opportunity, you’ll need to kill mature tap- or tuber-rooted or perennial weeds by consistently removing or treating foliage with herbicide as it appears. For grassy weeds such as Nutsedge, treat before six leaves appear. For broadleaf weeds, treat when leaves have just unfolded, but before they have time to start replenishing root stores.
If you use this technique on Dandelions, you’ll wipe out most plants with two treatments. With more stubborn weeds, this process could continue for multiple seasons, but don’t give up! Every time the plant pushes out new leaves that are killed, that effort is depleting food supplies in the root. At some point, food reserves will run out, and the plant will die.
Killing Weeds: Pulling vs. Spraying
Pulling weeds by hand is realistic when you’re dealing with a small area or just a few weeds. Weeds come up easiest when soil is moist and plants are small. If rain is scarce, water about 24 hours before weeding, soaking soil 6-12 inches deep.
Make sure you remove roots. With many perennial weeds, root pieces left in soil will sprout. Dandelion roots can extend to 24 inches deep, but most plants have roots 6-18 inches long.
Spraying herbicide plays an important role when you’re clearing vegetation from an area, are dealing with a large number of weeds or weeds you can’t eradicate any other way.
Prevent A Problem From Sprouting
Don’t let weeds gain a foothold. Use a pre-emergent herbicide to prevent weed seeds from germinating in planting beds and lawns. Don’t use pre-emergent weed killers in beds where you plan to sow seeds – the herbicide may prevent your seeds from germinating as well. Apply a layer of mulch to keep weeds from sprouting. In planting beds, a 2-inch-thick mulch layer can suppress weeds, as can tightly-spaced plants, which don’t give weeds the necessary elbow room, or sunshine, to survive.
Timing Is Critical
Attack weeds early in their growing seasons when growth is young and small. Don’t let annual weeds flower. If you don’t have time to pull plants, just yank blooms as they appear until you can finish the job.
If flowering does occur, don’t let plants disperse seed. One Dandelion produces an average of 15,000 seeds, which can live up to six years in soil; one Curly Dock plant produces 100-60,000 seeds, which can survive as long as 17 years. Allow one plant to cast its seed, and you’ll discover the truth of the gardening adage, “One year’s seeding means seven years’ weeding.”
In early spring or late fall (even after hard frost), perennial weeds can appear green, which may tempt you to apply herbicide. Don’t be fooled. When temperatures are low, plants aren’t actively growing. During summer’s hottest days, some weeds become dormant as well. Treating weeds with herbicide during dormancy is a waste of time and money. Watch for new leaf formation. That’s the clue to active growth – and the cue to fill your sprayer. For many perennial weeds, such as Dandelion or Bermudagrass, a late summer/early fall spray – just before plants enter dormancy – can prove effective. The chemical is transported to roots, which results in complete kill.
As some weeds, such as Crabgrass, mature, the leaves develop a hard coating that actually sheds herbicide. Even though the plant is actively growing, the herbicide can’t penetrate leaves. Crush or crumple leaves of mature weeds to ensure herbicide penetration. If you’re dealing with a patch of weeds, beat plants with a bamboo stake to tear leaves before spraying.
Specialized weeding tools do make it easier to remove weeds. Hoes wipe out young seedlings quickly, as do tined cultivators. Short knives fit between paver stones. Dandelion weeders work like a charm. Investigate weeding tools at garden centers or online to discover the right tool for your weedy situation.
If you choose to use a herbicide, be sure it’s labeled for the weed you’re attacking. Also check that the weed killer won’t harm your grass or surrounding plants you want to keep. Always read and follow label instructions carefully. To learn more about the different types herbicides, read Killing Weeds: Begin With The Basics.
Here are some more things you can do to get the best results from your weed-killing efforts.
- Create a spray zone by removing both ends of a can or the bottom of a 2-liter plastic bottle. Slip the container over the weed, insert the sprayer nozzle through the top opening, and spray. Use an open-ended cardboard box to target a larger weed patch.
- To spray one weed growing in the middle of desirable plants, cut a small hole in a large piece of plastic. Lay the plastic over the weed, pulling the leaves through the hole. Apply weed killer. Wearing gloves, remove the plastic after the herbicide dries. Handle plastic carefully to avoid dripping herbicide on cherished plants.
- Get pinpoint accuracy by using a paintbrush or glove to apply herbicide to specific leaves. If using a cotton or other type of porous glove, wear a plastic glove beneath it to avoid herbicide contact to skin.
- To spray weedy vines without harming garden plants, follow a step-by-step attack. First, cut the plant at soil level and insert a short stake near the base of the vine. When the vine resprouts, it will start to climb the stake. Before growth exceeds stake height, slip an open-ended plastic bag over the weed. Remove the stake, and spray the weed inside the plastic bag. Don’t remove the bag until herbicide has dried on foliage.
- If you accidentally splash herbicide on desirable plants, wash leaves immediately with water. If you are using systemic herbicide, prune affected plant parts immediately and wash remaining leaves with water.
After The Weed
Weeds are opportunists. Leave one bare spot, and multiple weeds will likely appear. Once you’ve dealt the deathblow to a weed, either by pulling, digging or spraying, fill in any resulting bare spot with mulch or seed. If you’re seeding, loosen but don’t turn existing soil, cover it lightly with compost and sow seed.