The tenacious ten Part 2

In this second part of a two-part feature, I’m covering the second half of my list of the 10 most common container weeds, discussing why they’re on the list and suggesting ways to combat them.

Weeds like to travel, they like the water, they like to wear disguises and they like to just hang out. Weeds in containers and container yards have done all their planning, have their ducks in a row and are now just chilling. Weeds just want to have fun.

The 10 most common container weeds are listed in Table 1 (below, on the right). These tenacious ten use four major strategies to get on this list. The acronym for the four strategies is DSWW.

• “Difficult to control” comprises issues regarding proper herbicide selection, hand weeding and pseudo-dormancies.
• “Sheer numbers” consists of extensive seed production abilities and effective dispersal mechanisms.
• “Weed seed continuum” involves the six lifecycles of weeds and which lifecycle(s) container weeds predominate, the corresponding timing of herbicide applications and the consideration of herbicide dose response within weed species.
• “We like it here” includes the heat and/or nutrient and water rich environment of the containers, which is just the place where weeds want to live and raise a family.
  
   

Creeping woodsorrel

Creeping woodsorrel (Oxalis corniculata) is a low-growing perennial that has long spreading stems that root out at the nodes. Creeping woodsorrel is a prime example of the “difficult to control” strategy that causes it to be one of the worst weeds to manage in nurseries.

Click the image above to view the full table.

Oxalis corniculata L. is a low-growing perennial weed which spreads rapidly by seed and has purplish-green long trailing stems. In a survey of 32 Alabama nurseries in 1987 by Gilliam et al. (1990), oxalis (Oxalis corniculata and/or O.stricta) and prostrate spurge (Euphorbia maculata), were two of the most difficult weeds to control. O. stricta is similar to O. corniculata in that both are perennials; however, O. corniculata has longer trailing stems that root where they touch the ground. Normally, Oxalis thrives in the cool season and behaves like a winter annual; however Cross and Skroch (1992) reported that oxalis can be a major problem year round under certain nursery production systems. Oxalis which is in the woodsorrel family (Oxalidaceae) has explosive seed pods similar to the mustards adding to its “sheer numbers” strategy. The leaves are compound and consist of three valentine-heart-shaped leaflets attached usually erect petioles by their pointed ends. This similarity to a clover leaf is why oxalis can be confused with black medic (Medicago lupulina L.) and white-clover (Trifolium repens L.) which are members of the legume family. O. corniculata will root out at each node and establish a new plant. If even a piece of stem with a node is left behind in hand-weeding, a new plant can establish. The flowers are bright yellow with five uniform shaped petals and 10-15 stamen.

For postemergent control of oxalis, Finale is effective and registered but must be used very carefully. Simpson et al. (2004) showed that diuron provided excellent oxalis control and that crop tolerance was good with dormant nursery crops. However injury occurred if crops were actively growing. Other researchers evaluating diuron as a preemergence herbicide have shown that nursery crop tolerance is improved with overhead irrigation soon after diuron application (1 hour) (Ahrens et al., 2003; Barolli et al., 2003; Richardson et al., 2004).

For preemergence control, James Altland (University of Oregon) found up to 60 DAT control with Snapshot (isoxaben + treflan) and Ronstar (oxadiazon). BroadStar (flumioxazin, Valent U.S.A. Corp.) also provided commercially acceptable control at 60 DAT (Altland, Oregon); however, the control with Snapshot and Ronstar was better. In a 2005 study at Ohio State University (OSU), we compared Showcase (Dow AgroSciences) an over-the-top 3-way granular-formulation [(trifluralin (2%) + isoxaben (0.25%) + oxyfluorfen (0.25%)] at 200 lb/ac (5 lb ai/ac, to Dimension (dithiopyr Dow AgroSciences) at 1.25 lb/ac (0.5 lb ai/ac), Gallery (isoxaben, Dow AgroSciences) at 1.33 lb/ac (1.0 lb ai/ac), Snapshot isoxaben + trifluralin, Dow AgroSciences) at 200 lb/ac (5 lb ai/ac), BroadStar (flumioxazin, Valent U.S.A. Corp., Walnut Creek, CA) at 150 lb/ac (3.75 lb ai/ac) and an untreated control to the Showcase On May 9, 2005, #1 pots were filled with soilless container mix consisting of 60% aged pine bark, 20% rice hulls, 10% sand, 5% technigrow, 5% stone aggregate and then treatments were applied. Efficacy pots were immediately seeded common groundsel (Senecio vulgaris), and creeping wood sorrel (Oxalis corniculata). All liquid sprays (Dimension and Gallery) were applied with a CO₂ backpack sprayer equipped with Teejet 8002 evs flat fan nozzles in a spray volume of 25 gal/ac (233.42 l/ha). Four species were evaluated for phytotoxicity: yew (Taxus ‘Everlow’), azalea (Azalea ‘Karen’), compact holly (Ilex glabra ‘Compacta’), and liriope (Liriope spicata ‘Silver Dragon’). Dates of evaluation were 15, 60, and 90 DAT (days after treatment). The efficacy trial indicated, Snapshot and BroadStar provided superior control of cxalis and groundsel at 15 DAT (Mathers and Case, 2005). Dimension gave poor control through all dates. Snapshot and BroadStar continued to provide commercially acceptable control across all species at 60 DAT (Mathers and Case, 2005). Showcase was commercially acceptable for oxalis at 60 DAT but not for groundsel. Starting at 15 DAT groundsel was escaping in the Showcase treatments. 60 DAT seemed to be the maximum residual for all five herbicides tested (Mathers and Case, 2005). BroadStar gave the highest efficacy scores but creeping woodsorrel was establishing at 60 DAT and pots were infested by 90 DAT (Mathers and Case, 2005). Liriope was somewhat phytotoxic to BroadStar and Showcase, although Showcase was less phytotoxic than BroadStar (Mathers and Case). Phytotoxicity was evident at 15 DAT and carried through to 90 DAT, although it was lessening with both treatments. There was some phytotoxicity with the other three species at 15 DAT, but it did not persist past 15 DAT, indicating there is little long-term phytotoxicity with any of these herbicides to Taxus, Azalea or Ilex (Mathers and Case, 2005). In recent studies we have also found Marengo G (indaziflam) (OHP) provides excellent control of oxalis (Mathers and Case, unpublished).
 

Annual bluegrass

Annual bluegrass (Poa annua) is commonly found in nursery containers and landscape beds. It utilizes the “weed seed continuum” strategy most effectively, capitalizing on the cool temperatures of fall and spring to inflict severe damage by nutrient and moisture competition with its dense mat-forming habit.

Poa annua has distinguishing light green leaves that are also distinctively boat-shaped at the tip and is a true winter annual, germinating in September to October. Each flower head produces thousands of seeds and it has a clumping habit.

Of the selective postemergence grass herbicides, Envoy (Clethodim) is the only one providing commercially acceptable control for bluegrass. Envoy is also effective on bermudagrass and tall fescue. Most of preemergence herbicides are effective and preemergence is the preferred method of control. In a trial conducted in 2002 and 2003 comparing Snapshot and BroadStar control for annual bluegrass, the BroadStar was very good in 2002 and statistically similar to Snapshot. However, in 2003, the BroadStar control was very poor while the Snapshot remained excellent. Besides Snapshot other excellent preemergence herbicides are Prodiamine, Pendimethalin (although some resistant populations exist), Dithiopyr, Oxadiazon, Oryzalin.
 

Birdseye pearlwort

Pearlwort (Sagina procumbens) is a member of the pink family with characteristic five, deeply dissected petals. Pearlwort can be an indicator of overwatering and utilizes the strategy of “we like it here” by optimizing on the moist conditions of containers and container yard. Pearlwort also uses the “sheer numbers” strategy, spreading easily by seed and rooting out at nodes, and every little piece left behind from weeding or in pot crevices or cracks.

Birdseye pearlwort is also known as arctic pearlwort and procumbent pearlwort. Pearlwort forms a dense mass of very narrow, hairless, pointed-tip leaves. The leaves are also opposite to one another along the stem, but appear to be arranged in whorls. It can flower from April to September and produces small inconspicuous white flowers. Birdseye pearlwort is a native perennial broadleaf plant that resembles moss and usually requires a disturbance to establish. It prefers wet, gravelly or sandy soils and is at home in the moist environment of containers and container yards. Thus, pearlwort is often an indication that irrigation is too frequent or excessive or the sites is improperly drained. Prostrate to ascending stems are up to about seven inches in length. Stems root at nodes and form a dense mat. Fruits consist of small capsules on fruit stalks that typically curve downward. Seeds can be spread by water splash, mowing, and on the soles of shoes. Although the reproductive potential from seed is extremely high, the greater threat to spread in nurseries has been from the procumbent growth habit and re-rooting at internodes.

Each little piece left behind in a crevice or crack of a re-used container creates a new plant. Every little piece needs to be removed in a hand weeding event and preemergence herbicides applied before any growth has occurred.

Some of the best preemergence products have been Rout (oxyfluorfen + oryzalin), and Snapshot is studies by Altland in Oregon. In studies by Judge, Neal and Wooten (2005) in North Carolina, Snapshot far out performed BroadStar or OH2 (oxyfluorfen + pendimethalin) (to a lesser extent). In studies in Connecticut, Mervosh and Ahrens (2008), BroadStar and Rout where the best performing herbicides at 16 Weeks after treatment (WAT). There are postemergence herbicides that work on Pearlwort but because of its perennial, mat-forming habit many require repeat application; in nursery stock preemergence control, proper watering and sanitation are essential to control.
 

Common groundsel

Groundsel (Senecio vulgaris) has two phenotypes making it the best example of the “weed seed continuum” strategy. Groundsel is in the aster family and as such has very effective wind dispersal of its seed. It is also a winter annual reproducing only by seed. Young plants are distinguished by their irregular lobed and toothed leaves that produce no milky juice. Older plants are easily recognized by their small conical-cylindrical, yellow, flower heads with black tipped involucral bracts around the base of each head. Although, a winter annual, groundsel perhaps is the best illustrator of the “weed seed continuum” strategy. Doug Doohan and his graduate student Rodrigo Figueroa at OSU, 2003, found that common groundsel plants alter their reproduction and seed dispersal strategy depending on growing conditions. Plants growing in autumn conditions favored dispersal in time. Fall plants produced relatively more dormant seeds that could survive unfavorable conditions for germination. Plants growing in summer conditions favored dispersal in space. Summer plants produced relatively fewer seeds that were non-dormant providing them greater potential for dispersal to sites where conditions were right for rapid regeneration. Temperature altered seed germination directly, providing a broad optimum temperature at which germination could occur and decreasing germination when temperatures were extreme. Temperature also affected the germination response of seeds indirectly by altering the physiology of seeds produced on plants exposed to varying growth conditions. Under warm condition 25ºC day/ 15º C night and 8-hour days and 16-hour nights, plants were tall in stature, produced more flowers, the seed ripened quickly and was non-dormant. However, under cool condition 15ºC day/ 5º C night and the same day-lengths as with warm conditions, plants were short in stature, produced few flowers, the seed ripened slowly and was dormant (Figueroa and Doohan, 2005). The two best herbicides for preemergence control of groundsel in Altland’s Oregon studies were Rout and BroadStar. As indicated above in the woodsorrel section, our OSU study with Groundsel indicated BroadStar and Snapshot were superior products and that Dimension and Showcase were in effective at control groundsel to 60 DAT (Mathers and Case, 2005).
 

Common chickweed

Chickweed (Stellaria media) likes the moist, high nutrient conditions of the ground fabric mats and drain holes that nursery container yards provided. As a member of the pink family as with pearlwort above the flowers have five petals that are deeply dissected. Chickweeds and stitchworts (Stellaria sp. and Cerastium sp.), are members of the pink family. Most preemergence herbicides work on members of the pink family, including Simazine, Diuron, Casoron, Surflan and Kerb. Many pink family members, however, are resistant to Ronstar. The best preemergence control for common chickweed is with SureGuard (flumioxazin). If SureGuard is used before chickweed plants are 2 inches tall, then postemergence control will also be successful. Glyphosate and Paraquat provide good postemergence control, but 2,4-D (which should never be used around ornamentals unless all other options for control are exhausted) provides good to poor control depending on the specific species. Common chickweed is often found growing out of the drain holes of containers growing on geotextile mats that cover the container yard or the floor of a polyhouse. High nutrient content, standing water and small amounts of media that flow with the excess irrigation out of the containers’ drain holes create an ideal environment for chickweed and illustrate this weeds “we like it here strategy.”

Common chickweed has white, small flowers that form at the tips of branches and in angles between pairs of branches. It is distinguishing from other chickweeds by ovate-pointed leaves. It is a winter annual that can flower throughout the season. It is a prolific seed producer. It is confused sometimes for mouse-eared chickweed which is often a perennial and has densely hairy stems and leaves. The leaves of mouse-eared chickweed are also sessile as with common chickweed stems are present.

These 10 common weeds utilize the “Difficult to Control,” “Sheer Numbers,” “Weed Seed Continuum” and/or “We like It Here” strategies for happy, prosperous lives in the nursery. Ralph Waldo Emerson said, “When nature has work to be done, she creates a genius to do it.” He also said, “What is a weed? A plant whose virtues have not been discovered.” With some inspiration from Emerson, I think weeds are geniuses at what they do and their virtues are for survival. And like Doug Larson says, “Weeds are plants that have mastered every survival skill except growing in rows.”

Hannah Mathers is a professor and extension specialist at The Ohio State University; mathers.7@osu.edu.

Did you miss the first part of this two-part series? If so, grab the May issue and flip to page 46, or read it here.