With the temperatures starting to fluctuate in late January, early February, now is the time to put down your pre-emergent. By the time soil temperatures reach 58 degrees, weed seeds will start to germinate. Why put down pre-emergent? Well, if you don’t, springtime weeds will be free to grow along with your grass and you’ll be fighting those weeds all spring long until the heat comes to kill them off.
What should I be putting down?
Pre-emergent works best in liquid form as it will provide the best coverage possible. Although harder to obtain for homeowners, a liquid concentrate that you mix to either spray with your hose or a pump sprayer will work best. However, you have to be precise in how much product you put out per 1000 square feet. Best to measure off 1000 square feet, fill 2 gallons of only water in your sprayer, and see how much you go through to spray that square footage. Likely, you will use between 1-2 gallons for the space. Once you know your spray rate, you can determine the correct amount in liquid or granular ounces to use. If you can contract this out to a company that has a tank sprayer, you’re likely better off as the concentrate will likely be as much or more than what you would pay for a single application from them. OrgLawn provides this so inquire if you’re interested in getting this done. On the flip side, with buying the concentrate, you’ll likely have enough for several applications.
Specticle Flo works great, Dimension is okay but there are many options to choose from. Some, are restricted use so you may not have access to everything unless you possess a commercial applicator license. Most homeowners use a granular form of pre-emergent. While this is not as effective as liquid, it is a lot easier. The reason why it’s not as effective is that when the granular gets watered in, there will be gaps in the coverage. Nitrophos makes a good pre-emergent but there are many to choose from. There are some pre-emergent products that contain Atrazine. While highly effective, it will take its toll on your tree roots and with enough use, it can actually kill your trees, especially younger trees. If you put it down regular pre-em heavy enough, you should have adequate coverage. Caution here. If you go too heavy, you’ll prevent your grass from performing like it should in the spring.
Always read application instructions carefully to determine the right application rate. There may be a limit to the amount of pounds per acre or 1000 square feet per year so again, read the label carefully. They put those words of caution there for a reason! Whether you choose a liquid or granular, you will have to water it in with at least ½ inch to get down into the soil.
How often should I do this?
Pre-emergent is usually best applied with split applications at lower levels. Say, put down February 1, then again 45-60 days later at a slightly lessor application rate. Durability is going to depend on not only the brand, rate, soil type, and timing, but also on how much rain or watering the lawn takes after the pre-emergent. The heavier the clay soil, the heavier you can likely go. The more rain you get, the shorter time span you can expect it to be effective.
Will this kill my weeds that are already there?
Most pre-emergent will not kill existing weeds. You will need to spray a post emergent selective herbicide designed to target your weeds at hand. Some pre-emergent will kill crabgrass in its early stages but for the most part, once weeds are showing, you will need to spray them separately.
Which weeds will pre-emergent stop from germinating?
This completely depends on the product you are using but in general, it should keep crabgrass, dandelions, clover, and other habitual offenders from sprouting. The more advanced formulas will keep nutsedge from coming but you have to go heavy to keep it from coming up. Nutsedge is incredibly hard to kill but you can slow it down by applying the right pre-emergent. The idea with applying a springtime pre-emergent is to stop a lot of the broadleaf spring weeds from coming. No pre-emergent is going to stop all of them and you may still get some weeds coming that are supposably controlled with the product you choose but the main purpose of the pre-em is help slow down the weed growth.
Can I still put down the pre-emergent after I see weeds?
You can still put it down but it is advisable that after you spray or apply your pre-em that you go back through with a post emergent and kill those weeds that are showing. If you don’t do this, even with the pre-emergent applied, you can get more weeds from the ones that are already there. Always apply your post emergent weed spray last as this has to dry on the weed leaf. If you spray your weeds first, and then apply a liquid pre-em right after, you’ll rinse off your weed spray rendering it worthless. Quick note about applying a post emergent weed spray. Make sure it’s selective meaning that it won’t kill your grass too (non-selective), and that the weed formula you are using is not going to harm your grass type. Some southern weed sprays cannot be used on northern lawns and or grass types. If you are using a concentrate to mix your weed spray, it’s best to add a non-ionic surfactant (depending on product check your label as it might not need it), along with vinegar.
If your water is high in alkalinity, you can add ½ ounce of vinegar per gallon to bring the mixture down to a 6.5-7.0 PH. This will make it a more effective killing machine. One final note about weed sprays, take note of the temperature thresholds on the label. Most over the counter products are going to have an upper threshold of 85 degrees. Any hotter than this in any part of the day and it will burn your lawn. On the flip side, the temperatures need to be high enough so that the weed synthesizes the product.
If it’s too cold, say under 60-65, it won’t be effective because the plant will not uptake the formula. Rule of thumb is that if the weeds are actively growing, it’s warm enough to spray them and should be effective. A second application 5-10 days later may be necessary for certain weeds.
Wait, won’t this hurt my soil microbes?
There are more soil biology friendly products like prodiamine that are not as harsh on the soil as other products. In my experience though, I have not seen a big performance hit, when used in correct amounts, to the grass growing season. The only thing that will greatly affect the microbes in your soil is the use of fungicides. Fungicides kill literally, everything good in your soil so even if you have brown patch, fairy ring, take-all patch, etc, try not to use it. Better to aerate it, put compost down, or another fungal based mixture that will bring your soil biology back into harmony.