Weeds have been resting all winter and can’t wait to shoot through and start growing. You probably already know that when they do, they start growing – well – like weeds! This often happens so fast that by the time you try to get rid of them, they’ve already taken over. Thin or bare soil areas combined with sunlight and warm soil temperatures create a perfect opportunity for germination of many of our annual weeds.
These weed seeds aren’t just perennials that lay dormant over the winter. They can be blown in by the wind or be carried in by our bird friends. Early spring is the time to start looking for these intruders and taking steps to reduce their spread. Below are some of the most common weeds we see in Minnesota. Along with our tips for getting them under control.
Crabgrass
Crabgrass is a summer annual that germinates inthe early spring and dies in the fall. The seeds germinate when soil
temperatures reach about 55°F. Crabgrass can often be found along sidewalks and driveways because the proximity to asphalt and concrete allows the soil to heat up much faster. Areas of the lawn that are south-facing and have full exposure to the sun may also see more crabgrass than other areas. It is a warm-season species that easily out-competes the cool-season grasses during the high-temperature stress periods of midsummer. It is easily recognized by its distinctive seedhead, which looks like protruding fingers, thus its Latin name Digitaria (digits, or fingers). The seedhead branches from multiple locations on the seed stalk. This grass has a rolled vernation, and smooth crabgrass can be purple near the base.
Crabgrass is generally controlled with preemergence herbicides that are applied before the seeds germinate. Organically, you can control crabgrass with an early and heavy application of corn gluten meal, aiming to get it down before the soil temperatures reach 55°F (3-years to achieve the best suppression). There is no organic post-emergent control for crabgrass, but synthetic herbicides containing Quinclorac can kill it.
Prostrate Knotweed
Prostrate knotweed looks a little like prostrate spurge, but it is easy to distinguish the two. The knotweed has symmetrical leaves that are arranged in an alternating pattern along the length of its runner. It also lacks the milky sap that is typical of the spurge. The knotweed germinates very early in the spring and can sometimes be found peeking through as the snow melts. It was identified at its earliest this year at the Turfgrass Research Center, in February! It looks a little like grass when it first emerges, and is sometimes mistaken for emerging crabgrass. The knotweed will usually show up six to eight weeks or even more, before crabgrass.
Prostrate knotweed can be controlled with pre-emergent weed control, but if it is applied early enough to control this species, it will lose its effectiveness before the later germinating species, such as goosegrass. Multiple applications of a synthetic post-emergent herbicide can have little effect. Knotweed is a sign of soil compaction, and it is usually found in compacted areas along sidewalks or in other heavily trafficked areas. Relieving the compaction through aeration and following other sound management techniques to produce a healthy lawn are generally the best way to handle knotweed.
Oxalis
Yellow woodsorrel (better known as oxalis and shamrock), is an annual or a weak perennial in northern regions of the United States. It is easily recognized by its three-part leaf with distinct heart-shaped leaflets and its yellow flowers with five petals. It germinates in midsummer and often becomes a problem in August.
This species can be controlled with pre-emergent herbicides, but its late germination makes it difficult to control in this manner. Selective post-emergent herbicides can control it, but these applications need to be applied carefully to prevent damage to desirable plants during the warmer part of the summer. Three to four applications of chelated iron would be needed to control it. This weed is easy to hand pull, as it does not resprout from roots left behind. Proper irrigation, fertilization, and other sound management steps that keep the lawn healthy provide the best means of preventing this weed from becoming a problem.
Black Medic
Black medic is considered an annual clover, even though it is not part of the clover
genus. It has teardrop-shaped leaves clustered in groups of three, similar to clover. The flowers are small, round, and yellow. Black medic is a species that is an indication that the lawn grasses are not as competitive as they could be with proper management. It often shows up on hillsides, where water does not readily penetrate the soil, or in compacted areas.
The adoption of sound lawn management techniques, along with pre-emergent or post-emergent weed control, will provide satisfactory control of this species. Hand weeding this weed is not difficult, since it grows from a central location. One application of chelated iron will kill this weed.
Prostrate Spurge
Prostrate spurge, as its name would indicate, grows in a prostrate manner along the surface of the ground. It has leaves that are paired opposite one another along the length of its runner. The individual leaves are asymmetrical, meaning that they are not the same on both sides of the midrib. Some varieties will have a purple/red spot on the middle of the leaf. If the stem is broken, a drop of milky sap will form at the end of the broken stem. This sap can irritate the skin, so wear gloves when hand pulling this weed. Spurge and knotweed can be mistaken for each other, but knotweed does not produce a milky sap when the stem is broken and its leaves grow in an alternating pattern. Spurge generally germinates later than goosegrass, which germinates later than crabgrass.
These three species are a difficult combination to control with pre-emergent herbicides. If the herbicide is targeting crabgrass germination, it will have lost its effectiveness before the spurge germinates. Spurge can be controlled with a number of post-emergent herbicides, such as 2,4-D and MCPP, but it is risky to use these chemicals in midseason because of damage to trees and shrubs. Spurge generally germinates into open areas in the lawn, and the best way to control it is with proper cultural techniques that prevent it from becoming established.