Spring is approaching and the main thing on my mind is controlling annual weeds. The most effective way to control crabgrass and other summer annual weeds is with a pre-emergent herbicide. Pre-emergent herbicides should be applied prior to weed seed germination. They don’t prevent germination, but they do prevent emergence of shoots and roots by forming a barrier on the soil surface.
Summer annuals such as crabgrass and chamberbitter begin to germinate when soil temperatures warm in the spring. Preemergence herbicides should be applied when daytime temperatures reach 65oF to 70oF for 4 or 5 consecutive days to form a barrier to help prevent these weeds from emerging. This is about the same time azaleas and dogwoods begin to bloom. Goosegrass is the exception for this temperature rule. For good goosegrass control, preemergence herbicides should be applied 3 to 4 weeks after the suggested daytime temperature application date.
It is important to note that you should only use preemergence herbicides on lawns that have been established for at least a year. These herbicides are prone to injure newly planted lawns. In addition, many preemergence herbicides may interfere with lawn grass seed germination, so make sure to refrain from reseeding for at least six after application.
Commonly available preemergence herbicides contain the active ingredients oryzalin, benefin, pendimethalin, DCPA and bensulide. However, there are a wide variety of products on the market. For more information on weed management please read the UF/IFAS publication “Weed Management Guide for Florida Lawns”.
Weeds can be the bane of the gardener’s existence and Extension Agents get a lot of questions on how to manage them throughout the year. Whether you are growing edibles, ornamentals, or turfgrass you have probably encountered a plant out of place which is basically the general definition of a weed. Just like with any other landscape challenge the first thing you need to do when dealing with weeds is accurate identification. Understanding the life cycle of the pest (weed in this case) you are targeting will be key for the most effective control with minimal inputs. Your local Extension office can assist with weed identification, and you can find a list by county here.
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Weed control is never ending in Florida landscapes. If there is a bare area of soil, weeds take advantage of that void and make themselves at home. Ideally, we would use good cultural practices that minimize weed invasion, but if prevention is ineffective and you need to use an herbicide, make sure you are using that tool properly.
When I talk to homeowners and commercial applicators about weed control, many times they have selected the appropriate product but are still struggling with management. While troubleshooting the problem, many times I discover, the reason is related to poor uptake because the client is not making herbicide applications to what I like to call “happy weeds.”
When using an herbicide, you need to think about how it works. Not the deep level chemical reactions, but rather consider how the active ingredient is going to be delivered to whatever physiological function of the plant it is targeting.
Does the herbicide need to be absorbed by the leaves/stems/roots?
Once absorbed, does it need to travel through the vascular system of the plant (translocation)?
What effect will temperature, moisture, mowing/trimming have on product uptake?
All these factors are very important because if a plant is stressed, the primary response is survival. During excessively hot weather plants close off stomata, form waxier surfaces, and do everything they can to retain moisture – which also includes reduced absorption of herbicides. In cool weather, the plant may be dormant or have slowed growth that will reduce translocation.
Mowing and trimming reduces leaf surface area minimizing uptake. Overcoming injury also triggers the plant to go into water conservation mode which also limits product uptake.
So, what do I mean by “happy weeds”? For herbicides to be effective, they should be applied when growing conditions are ideal for your target plant. Optimum soil moisture, soil temperature, ambient temperature, and minimal stress lead to “happy weeds” that are primed to accept herbicides and translocate if needed. Plants in survival mode will have their defenses activated and this decreases herbicide efficacy.
Always read and follow entire label instructions of all pesticides.
Do you have a low-growing weed that is producing tufts of white, fluffy, dandelion-like seeds, which float in the wind when disturbed or mowed? This is Annual Trampweed (Facelis retusa). I did not see this weed in North Florida until recent years. It’s native to South America.
Mid-April to early May is the time of year when this winter annual weed goes to seed in North Florida. That’s what it is doing now and it is a prolific seed producer. Each white tuft contains numerous seeds. Each tiny seed is attached to a small individual bristle, coming out of the larger tuft, which is carried by wind. This allows hundreds of seeds to move to other locations.
Trampweed is approaching the end of its life as we move into early May. As a winter annual, the individual weed dies in response to warm temperatures only to leave behind hundreds of seeds that survive the summer. These same seeds come up the following fall to early winter to begin the next generation. The best time to attempt chemical control with an herbicide is well before these weeds mature and begin flowering.
One chemical control option is to apply a lawn preemergence herbicide during October when nighttime temperatures drop to 55° to 60°F for several consecutive nights. This will be just before these winter annual weeds emerge. Done correctly, the application of a preemergence herbicide forms a temporary chemical barrier along the soil surface preventing the winter annual weeds from emerging. Hence the name preemergence.
A second application of preemergence herbicide may be required six to nine weeks after the initial application to achieve season-long control, based on the product’s label directions.
If you miss this narrow window of opportunity to apply a preemergence herbicide, watch for the small young weeds in winter and treat then with a postemergence herbicide that is labelled for use in the type of lawn grass you are growing.
Waiting until trampweed is producing flowers and seeds in April and May to attempt control is almost worthless in controlling this weed. Correct timing is critical.
Trampweed usually is found in areas of a lawn that are already weak and thinning. It favors open, dry, stressed and low-fertility areas of a lawn. So, try to manage your lawn correctly. This involves learning to mow, irrigate and fertilize correctly for the type of lawn grass you are growing.
Hopefully, you are enjoying No Mow March. With the extra time provided by not mowing, you can spend more time trying to observe the diverse array of flowering plants coming up in your lawn. Often these are considered weeds, but a weed is just a plant you don’t like growing where you don’t want it. However, if we can find some beauty in these plants and appreciation for their role in supporting wildlife, then maybe we can turn it around and start thinking of them as wildflowers. As a trained botanist, I’m interested in all the flowers and think of how they grow, what eats them, what pollinates them, how their seeds are dispersed, and what niche do they fill in the ecosystem, even if that ecosystem is my residential landscape. Below are a couple of my favorite plants that come up in my infrequently mowed and/or unmowed lawn during the early spring.
Early Violet (Viola palmata)
This is the one that got me started leaving areas of the yard unmowed or infrequently mowed. Those violet-colored flowers were just too pretty to mow over, and I began giving them a wide berth. Now, I have little meadows of these native violets that spring up every year and give me great pleasure to observe.
Earthsmoke (Fumaria officinalis)
Not only is this one pretty, but it has one of the coolest common names of a plant that I know of. This non-native relative of the poppy plant has delicate foliage, reminiscent of the native columbine, with flowers mostly a light purple, but with a touch of dark purple towards the tips. The flowers also have a strange little twist to them. Part of my fondness for this plant is also tied to a memory from college. I was learning how to use dichotomous keys at the time and was so proud to have successfully keyed this one out. My identification was confirmed by our taxonomy professor, who I greatly admired, and I think he was a little proud of me, too!
This one is related to earthsmoke, but in a different genus and just happens to be a native. It also has similar delicate foliage and a twist in the flowers, but these come in yellow.
Mock Bishopsweed (Ptilimnium capillaceum)
This native carrot family relative, with small flower clusters and delicate, wispy leaves, is not one that jumps out at you as being super showy. However, being a carrot relative, it is a great larval host plant for the swallowtail butterflies.
Horrid Thistle (Cirsium horridulum)
The common name says everything about how most people view thistles. Thistles definitely get a bad reputation due to those spiny leaves, but our six native species are a boon for pollinators. Being in the Aster family with many flowers grouped together, it provides lots of easily accessible pollen and nectar resources. This one is also a known host plant for the little metalmark and painted lady butterflies, which somehow find their way around all of those spines. I really enjoy the artistic symmetry of the leaves, and also the spines that surround the flower in bud, especially when the dew has collected on it and the light hits it just right.
Southern Dewberry (Rubus trivialis)
Probably another hard sell for leaving in the yard, but this prickly native plant rewards animals, including us, with delicious fruit. This one is best left in a low trafficked area where it won’t grab you.
Venus’ Looking Glass (Triodanis perfoliata)
This native beauty ties for second coolest common name, which is a reference to the shiny covering of the seed. Related to bellflowers and lobelias, Venus’ looking glass shoots up in the early spring and has beautiful purple blooms atop a wispy inflorescence (flower stalk). You’ll find several pollinators visiting this lovely little wildflower.
No Mow March is a perfect opportunity to spend time learning more about the non-turf species living in your lawn. There’s a lot more than the list above and your local extension office can help you identify what you’re seeing. You can also use the iNaturalist app for help in identifying these little pretties. If you use iNaturalist during March, please add them to our No Mow March project. For more information on No Mow March, please visit https://sfyl.ifas.ufl.edu/escambia/lawn-and-garden/no-mow-march/.