A thick lawn is your best defense.
Weeds are opportunists. They will find bare spots or places where your grass is weak, and they will exploit them to their advantage.
Perennial weeds (weeds that grow from their roots every year) can spread and make a lawn unsightly. Annual weeds (weeds that die at the end of the season and reseed the next year) can leave bare spots that are vulnerable to runoff.
No matter what weeds you have, the first line of defense is preventive practices. Try these options to get at the root of the problem first, before resorting to herbicides.
Mow high. Do not mow grass shorter than recommended for the species you grow. Mowing at 3 inches or higher helps grass shade out weeds and encourages a thicker, more competitive turf. See other sections of this site to make sure that you are using the right grass species, fertilizing and watering correctly, and generally doing all you can to encourage healthy grass.
Reduce compaction. Pay special attention to heavily used areas and sections next to pavement. Weeds can gain a foothold in these spots and spread to the rest of the lawn if it is weak.
Repair bare spots by raking in seed in early spring so that the new grass can compete with the weeds that are sure to come up. This can be tricky though. When you seed, you can’t use traditional pre-emergent crabgrass products because these will keep your grass seed from germinating just like the crabgrass seeds.
There are however a couple of products and strategies to avoid this situation and keep the spring crabgrass germination
If lawn is thin, fertilize it properly ,with quality fertilizers ,to improve density.
Let the weeds be your guide. If weeds dominate an area, it’s likely that there is something wrong with either the growing conditions or your lawn practices. Dense stands of prostrate knotweed are a good sign of soil compaction. Don’t just pull out the weeds. Relieve the compaction. Violets (Viola spp.) are a good sign of low light levels. One solution might be to seed shade-tolerant fine fescues or new shade and drought tolerant hybrid bluegrasses.
If you use herbicides…
• Use the right product at the right time. Follow label directions and try to spot treat areas with the weeds only using the right liquid concentrate weed control. The best and most economical way is in a pump sprayer. You usually mix a very small amount with water and spray. This saves lots over time.
• Use granular weed control products only on lawns with lots of weeds throughout. Some products are better than others for certain types of weeds. Applying at the right time, and allowing the weeds to take in the weed control is critical. Usually this is done while the grass is wet or damp to help the granules stick to the weed. 24-48 hours without rainfall is best.
• To avoid volatilization and drift, which release pesticides into the air, do not spray when temperatures are high or it is windy.
• To help prevent polluted runoff, do not apply pesticides when heavy rains are expected or the ground is already saturated or frozen. You will also get a better result.
• Sprayers should be triple rinsed with a spray tank cleaning solution to avoid residual left over when you use the sprayer for other products.
The types of weed control products include:
• Most common for crabgrass.
• Applied to soil before weeds are expected.
• Have low solubility and bind to organic matter.
• Most common for perennial broadleaf weeds.
• Applied after weeds have emerged and are actively growing.
• Avoid application before irrigation or rain.
• Kill or injure all plants they come in contact with.
• Used to kill vegetation before reseeding.
Annual grass weeds.
Crabgrass is one of the most common grass weed problems. It is a warm-season annuals. They thrive when temperatures are hot and cool-season lawn grasses are least competitive. Still, they have a tough time invading a healthy lawn.
One place where they can more easily gain a foothold is along paved areas where high temperatures can damage cool-season grasses – along the edges of driveways, sidewalks and patios, for instance. Soil temperatures are usually warmer in these areas and crabgrass germinates earlier. These are also harder to get granular applications on as you are spreading your product in a spreader.
Where hostile conditions exist for lawn grasses, you can spot treat for crabgrass with pre-emergence herbicides. These herbicides work on the seeds as they germinate. Because they are ineffective on ungerminated seeds or established plants, timing is critical.
Using a strategy of spraying just the edges of the driveway or sidewalk about 1-2 ft wide, will keep crabgrass pressures down considerably. The benefit is great, it doesn’t cost much, and you are only treating a small area along the edges where crabgrass pressures are greatest.
Optimum timing for pre-emergent treatment of crabgrass is about the time that forsythia blooms wane, when the soil temperature is between 59 F and 65 F.
As mentioned earlier, Pre-emergent herbicides do not distinguish between weed seeds and grass seeds. So you won’t be able to replant grass where you’ve applied them for 2 to 6 months. Two products do exist to allow you to seed in spring and control crabgrass. Professionals use them and you can too.
The first product is called Siduron. It is usually easiest to apply this as a granular over the seeded area at the time of seeding. It won’t inhibit new grass seed from germinating while controlling crabgrass. Siduron is a little pricey, but their aren’t exactly many alternatives.
The second product is Drive DF. It is a dry flowable product that you mix in water and spray before you seed an area. It works great in small seeded areas because you can spray it where you want to seed. You use only about 1/3 of an ounce per gallon of water. The best part: It is also a post-emergent crabgrass spray too. You can use it to spray existing crabgrass plants if some emerge anywhere else in your lawn. It also controls a few broadleaf weeds like clover too. It can be bought in Drive 1# containers for a little more than 100 dollars (professional s use cases of this size) . It can also be purchased in Drive 1.5 oz bottles for about 20 dollars. This size will make 5 gallons of crabgrass pre-emergent for seeded areas or crabgrass killer for mature crabgrass plants .
As mentioned above, once crabgrass emerges, you can apply postemergent herbicides, usually from early June through mid-July. Several different herbicides are on the market that can kill plants that have not yet tillered. Drive DF is a good one . Acclaim Extra is another good product. Acclaim Extra is only a post-emergent crabgrass control . It is a liquid you mix in water and spray on crabgrass. It comes in large size concentrate, but is also sold in Acclaim pint size containers. An average rate is about ½ oz per 1000 sq ft or gallon of water. This will give you 16 gallons or 16,000 sq ft of crabgrass killer.
Spot treating with non-selective herbicides such as Round-up can kill the plants and reduce their contribution to next year’s seedbank. But you must use absolute caution and care not to accidentally spray and kill other plants nearby. Round Up will also kill any grass it touches and leave dead spots throughout the lawn. Drive DF and Acclaim Extra will not do this.
Perennial broadleaf weeds
Unlike annual grass weeds, herbicides for broadleaf perennial weeds are usually applied post-emergence. The advantage of post-emergent control is that you can see how many weeds you have before you decide whether or not to spray. If you just have a few, pulling them by hand might be your best option. If you don’t have to spray, then don’t.
Most broadleaf perennials – such as dandelions — have their greatest visual impact in spring. But late summer to mid-fall is a great time to control them with herbicides. As the weather cools, these weeds start storing food produced in their leave in their roots, just like cool-season lawn grasses. If you apply herbicides at this time, it will be transported along with the food and stands a better chance of killing to root.
When applied in spring, you can still get good results with quality weed control products. Because the weed is hungry and growing, it will take the weed control in and be effective at this time too. You can spray them with the quality weed control products, or use granular weed controls in a spreader. Spraying is more economical and you get the product right where you want it. Granular products are more suited for large areas filled with weeds to get a knockdown. Avoid rainfall for 24-48 hours. This gives the weed control time to work.
Make sure you choose a selective broadleaf herbicide – one that kills only broadleaves and not grass. Nonselective herbicides, such as Round-up, can kill all plants that they come in contact with.
I will detail some more specific weed control strategies as the season progresses, but this should get you on your way.
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