How to Kill Crabgrass
The lawn weed, crabgrass (Digitaria spp) is a warm-season annual weed, which means it reproduces by seed. People often ask me how to kill crabgrass. The short answer: applying preemergent herbicides at the right time is the best way to kill crabgrass.
Preemergent Herbicides for Killing Crabgrass: When to Apply
To get rid of crabgrass, it helps to know its life cycle. When spring soil temps (at a depth of 2″-3″) reach 55-60 degrees Fahrenheit, the first crabgrass (also spelled “crabgrass”) seed will germinate. From mid-summer to fall, crabgrass produces seed. The crabgrass plants (but not the seeds) are killed by frosts in autumn.
Preemergent herbicides (also spelled “pre-emergent”) come in either granular or liquid form and kill crabgrass seedlings as they germinate. Think of preemergent herbicides as forming an invisible shield across the soil surface that stops emerging crabgrass dead in its tracks. This shield image will serve as a reminder not to practice core aeration on lawns after applying preemergent herbicides since doing so would only “puncture” the shield. Aerate lawns beforehand, instead.
As their name suggests, preemergent herbicides kill crabgrass at a specific time: before its seedlings emerge. For success in getting rid of crabgrass in this manner, timing is of the essence. Apply pre-emergent herbicides before germination, but not too far ahead. Crabgrass germination coincides approximately with the blooming of Redbud trees, Bridal wreath Spirea and Forsythia.
Types of Preemergent Herbicides for Killing Crabgrass
There are many different types of preemergent herbicides for killing crabgrass. “Weed and feed” products often contain preemergent herbicides, although some question whether their concentration is strong enough to be effective. I will focus on 2 preemergent herbicides: Dimension and Tupersan.
Dimension (active ingredient, dithiopyr) is safe to use on most lawn grasses (check the label first) and provides long-lasting coverage — an important consideration, since not all crabgrass seed germinates at once. Thus Dimension will kill later-germinating crabgrass, too. Dimension also displays some effectiveness as a post-emergent herbicide.
Tupersan (active ingredient, siduron) is worth mentioning because, unlike other preemergent herbicides, it will not damage germinating lawn grass seed. In fact, its active ingredient is often combined with starter fertilizers. Alternatively, for newly-seeded lawns, wait until after 3 mowings (or 3 months, to be on the safe side) before applying preemergent herbicides.
Using Preemergent Herbicides for Killing Crabgrass: DOs and DON’Ts
Do:
Irrigate afterward: water activates preemergent herbicides.
Re-apply preemergent herbicides, if you question your product’s coverage. Because crabgrass seedlings do not all germinate at once, re-application can kill some of the later-germinating crabgrass.
Follow label directions and apply the proper rate. Measure the lawn area and calibrate your spreader carefully.
Irrigate afterward: water activates preemergent herbicides.
Re-apply preemergent herbicides, if you question your product’s coverage. Because crabgrass seedlings do not all germinate at once, re-application can kill some of the later-germinating crabgrass.
Follow label directions and apply the proper rate. Measure the lawn area and calibrate your spreader carefully.
Don’t:
Dethatch or aerate the lawn after applying preemergent herbicides.
Apply pre-emergent herbicides on new sod.
There are also post-emergent herbicides for killing crabgrass well after it has germinated (e.g., Acclaim Extra), but they are effective only at killing young crabgrass plants. Because these young crabgrass plants are small, they are very difficult to detect in a lawn — rendering post-emergent herbicides far less useful than preemergent herbicides for killing crabgrass.
Crabgrass Control: Organically
“But do I really need chemicals for crabgrass control?” you ask. The answer is, No. While the best way to kill crabgrass is with preemergent herbicides, the best way to control it is by having a healthy lawn. Here are some ways to promote lawn health — at the expense of crabgrass:
Fertilize more heavily in autumn than spring. By autumn, frosts will have already killed any crabgrass.
Don’t let bare spots remain uncovered for long, else opportunistic crabgrass will take root. In the fall, fill in those bare spots by overseeding.
When irrigating the lawn, water more deeply and less frequently. Crabgrass is a notoriously shallow weed.
Mow “high”, leaving the lawn grass at a height of 2 1/2″-3″. This will allow the lawn grass to “protect its own turf” better, depriving crabgrass seeds of the light they need to germinate.
Final Notes on Crabgrass Control
A great organic “weed and feed” product is corn gluten. An organic preemergent herbicide, corn gluten will suppress crabgrass germination, while fertilizing your lawn.
Don’t forget good old weeding as a method of crabgrass control. Hand-pulling small patches of crabgrass before it goes to seed makes eminent sense. To facilitate weeding, water the lawn first (weeds are more easily extricated from wet soil).
Question: Jimmie, My Knockout Roses looked just wonderful a few weeks ago when we were getting all that rain. Now they are not blooming much at all and just have lots of dead old bids on them? Do I need to be doing something else to them? Thank you for your time. Ginger L. in Prosper.
Answer: Hi Ginger, a few things to point out. One you need to be deadheading those old blooms off your Roses whenever possible, the nutrients in the soil feeding your Roses are still going to those old blooms instead of producing new blooms. Second, have you ever fed your Roses? It is important to put good nutrients back into your soil at least once if not twice a year to continue good feeding on all your landscaping plants, trees, shrubs, lawn etc…
Plants are no different than we are, if you go to your fridge at home a few times a day and get food out for yourself to eat eventually you will need to go to the store and buy more food to put in your fridge right? Once a plant uses all of its nutrients it will no longer thrive and start looking tired and hungry just like us! Until next time…Happy Gardening!!