The annual deep purple bloom of Purple Deadnettle (Lamium purpureum) and Henbit (L. amplexicaul) in Ohio farm fields and a purple haze rising over lawns may conjure a nostalgic pop culture smash-up. These non-native showy weeds belong to the mint family, Lamiaceae, as evidenced by their square stems which is a family trait.
Henbit has scalloped leaves that are evenly spaced along the stem. The drooping triangular to heart-shaped leaves on deadnettle make plants look like miniature pagodas.
Adding to the colorful bloom display are the mellow yellow flowers of Cressleaf Groundsel (Packera glabella, formerly Senecio glabellus). This toxic native plant belongs to the Aster family, Asteraceae. Its membership in the family is revealed by seed heads that look like miniature versions of the puffball seedheads of Common Dandelion (Taraxacum officinale).
Cressleaf groundsel is so-named because its lower leaves resemble watercress. Its alternate common name of butterweed comes from its conspicuous buttery yellow flowers.
Common Chickweed (Stellaria media) belongs to the pink family, Caryophyllaceae. The name of the genus, Stellaria, is derived from “stella” meaning “star” and references the star-shaped white flowers. However, it’s the creeping, mat-like growth of the mature plants rather than the tiny flowers that draw our attention to common chickweed plants.
Hairy Bittercress (Cardamine hirsute) belongs to the mustard family, Brassicaceae. The family’s accepted alternative name is Cruciferae with “cruc” referencing the cross-shaped flowers (Latin “crux” = cross), and “ferae” meaning “bearing.” It’s a prolific seed producer with seeds developing inside vertical pods at the tops of the plants. Ripened seed pods pop open at the slightest disturbance to launch the seeds several feet.
The Winter of Our Discontent
These spring bloomers all have one thing in common: they are winter annuals. The graphic below shows why I consider winter annuals to be sneaky weeds.
The seeds of winter annuals germinate in late summer to early fall. Plants grow throughout the winter and flower in the spring. Plants die in early summer. The timing can make landscape and turfgrass managers believe a post-emergent herbicide application killed the plants when the plants actually died of natural causes.
The seeds of summer annuals germinate in the spring and plants grow throughout the summer to flower and produce seeds in the fall. The vast majority of our pre-emergent herbicide applications are made in the spring.
This is understandable given that some of our most notorious lawn and landscape weeds have summer annual life cycles. A spring application of a pre-emergent herbicide can indeed be effective in suppressing Crabgrass (Digitaria spp., family Poaceae), Yellow Foxtail (Setaria pumila, family Poaceae), and Spotted Spurge (Chamaesyce (=Euphorbia) maculata, family Euphorbiaceae). However, spring pre-emergent herbicide applications have no effect on winter annuals.
A Word From Management
The winter annuals highlighted in this Alert may present a problem in both lawns and landscapes, with the exception of cressleaf groundsel. This weed is easily kept in check in turfgrass by mowing. Landscape management includes cutting or post-emergent herbicide applications made before plants produce flowers.
David Gardner (Professor - Turfgrass Science, OSU Horticulture and Crop Science) provided a helpful “Winter Annuals in Turfgrass 101” lesson last week during our BYGL Zoom Inservice. Dave noted that many of the same pre-emergent herbicides used against summer annuals in turfgrass are also effective in suppressing winter annuals.
These include dithiopyr (e.g., Dimension, Quali-Pro Dithiopyr 40 WSB), pendimethalin (e.g., Pendulum, Scotts Halts Crabgrass & Grassy Weed Preventer, etc.), and prodiamine (e.g., Prodiamine 4L, Barricade, etc.). However, applications should be made in late August to early September as illustrated in the winter annual life cycle graphic.
Of course, it’s important to remember that the winter annuals highlighted in this Alert are considered opportunistic weeds. They take advantage of openings in turfgrass. Thus, the best long-term strategy for suppressing opportunistic winter annuals in turfgrass is to focus on thickening the turfgrass which may involve over-seeding.
A Word of Caution
If over-seeding is planned, it may be best to avoid making a pre-emergent herbicide application. At the very least, it’s critical to read and closely follow label directions to make certain the herbicide doesn’t prevent the establishment of a thick, competitive stand of turfgrass.
For example, here is a quote from the Dimension 2EW label: “Newly established turf must have developed a good root system and a uniform stand and have received at least two mowings following seeding or sprigging before making the first application of this product.” Likewise, pendimethalin and prodiamine product labels commonly include such cautionary statements relative to newly seeded turfgrass.