The tenacious ten

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

The 10 most common container weeds are listed in Table 1. 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.
   

The top 10 weeds will be listed according to which D, S, W₁ or W₂ strategies best illustrates their rise to predominance in nursery containers. In this article I present the first five of the top 10 most common container weeds. In part two I’ll discuss the remainder of the list.

Click image above to view the full Table 1.

Arguably, all the weeds listed in Table 1 could be called “difficult to control.” However, I have reserved the “D” or “difficult to control” strategy to those weeds that have a particular issue with physical or chemical controls, improper herbicide selection or resistance issues. Therefore only the two bittercress, the liverwort, perennial oxalis and annual bluegrass have a “D” strategy.

Every weed in Table 1 is extremely effective at either sexual or asexual propagation. Often the “sheer numbers” strategy is a requirement of any successful weed population. “Sheer numbers” has two adaptive advantages. The first is a matter of reproductive survival, with so many propagules generated you virtually guarantee some will live to reproduce another generation. The second advantage is a matter of adaption, by reproducing in large numbers, the probability of finding that one rare individual with some selective trait for herbicide resistance, drought tolerance, handling compacted soil, or some other desired ability is significantly increased.

The ten common container weeds presented here span the gamut of life cycles -- filling different niches on the nursery production calendar to maximize the range of the “weed seed emergence continuum.” Container weeds emerge from early spring with the summer annuals such as chickweed, through mid-summer with the prostrate spurge, into fall with the winter annuals such as annual bluegrass and bittercress, and the perennials “somewhat ever present” depending on your region of the country with liverwort in winter or creeping oxalis in summer. The “we like it here” strategy with container weeds usually involves the appeal of the well watered and nutrient rich environment that the container provides. Some weed species such as common chickweed and pearlwort have even taken this strategy a step further to include the intersections of the container drain holes and the ground fabrics. Here these species enjoy the water-nutrient, media-laden solution that pours out the bottom of the container after each irrigation or rain event. The “we like it here” strategy for other species, however, is all about the heat. Spotted spurge is a good example of this thriving where temperatures are “smoking hot,” it finds its niche in the container yard, in crevices of poly houses and of course the black plastic container itself.
 

The bittercress species

There are many different species of bittercress. However, the two species seen most often are the hairy bittercress (Cardamine hirsuta) and the Pennsylvania bittercress (C. pennsylvanica). Hairy bittercress is an introduced species with basal leaves evenly disbursed within a rosette and is a true winter annual. As its name implies, it has hairy leaves and the leaves are also rounded. Pennsylvania bittercress by contrast is a native species with erect or spreading stems, with no hair and the leaves are pointed. Although usually a winter annual, the Pennsylvania bittercress can take a biennial habit. All bittercress are members of the mustard family and thus have dehiscent seed pods, which is why another common name for Cardamine sp. is snapweed. It is this snapping property of the seed pods that makes it very difficult and expensive to control with hand weeding.

Isoxaben (Gallery) and imazaquin (Image) are recommended for postemergence control of Cardamine hirsuta, although Gallery was not as phytotoxic as Image on the plants evaluated. Research demonstrated that Gallery provided excellent post-emergence control of hairy bittercress with no injury to a broad spectrum of woody ornamentals, and that control was influenced by size/age of the weed. Small non-flowering bittercress were controlled with 1.0 pound active ingredient per acre, while 2.0 pounds were necessary to control large, flowering bittercress. Pre-emergence control is the best way to control Cardamine sp. James Altland (Oregon State University) in studies pre-dating some other the newer preemergence products such as FreeHand, Marengo and Biathlon found excellent control of Bittercress with Snapshot (Gallery + treflan). Joe Neal (North Carolina State University) in 2007 trials, found FreeHand (dimethamid-p + pendimethalin) was as good as or better than Snapshot in controlling bittercress. In a 2011 trial, Charles Gilliam (Auburn Univeristy) also showed Biathlon (Goal + pendulum) at 200 lbs/ac provided excellent bittercress control. Bittercress is one of the weeds that uses all four strategies defined in this article. It is an excellent example of a species that relishes the moist environment of a container. It is difficult to find Cardamine sp. listed in traditional field weed control identification books, because it really is a marsh environment species, more native to wetlands than to agricultural or horticultural environments. Again, the water-rich environment of the container and the container drain holes makes this weed say “we like it here” and we want to stay.
 

Prostrate/spotted spurge

Prostrate spurge (Chamaescyce maculata or C. humistrata) in nursery containers is one of the most competitive weeds. Growing in Oregon nursery containers, spurge resulted in significant growth and quality reductions in Azalea ‘Rosebud’ and ‘Gold Cone’ common juniper. Even though it is a “difficult” weed, I think the best strategy that prostrate/spotted spurge uses is the “weed seed continuum.” It dominates containers in mid to late summer. Optimum conditions for spurge germination include temperatures of 25-30 ºC and light. Prostrate spurge germination is also influenced by fertilizer placement, methods that limit nutrient availability in the top surface of the container reduce spurge establishment, indicating its effective use of the “we like it here” strategy. After incorporating controlled release fertilizers (CRFs), Ruter and Glaze (1992) reported 96 percent and 86 percent control C. humistrata 8 and 12 weeks after treatment (WAT), with combinations of the herbicides, Ronstar (oxadiazon) + Surflan AS T/O (oryzalin). Whitwell and Kalmowitz (1989), however, after topdressing CRFs found that C. humistrata control with combination herbicides was 59 percent and 52 percent control 8 and 12 WAT, respectively. Altland and Fain (2003) speculate that fertilizer placement may explain some of the discrepancy between results in these two studies.

Other researchers have found only combination herbicides provide spurge control after 30-45 DAT (Fare and Robinson, 2001; Judge and Neal, 2000). Of five herbicides that provided effective control at 28 DAT, only Gallery (isoxaben) + Surflan was providing spurge control at 70 DAT (Judge and Neal, 2000). Fare and Robinson (2001) found OH2 (oxyfluorfen + pendimethalin), provided that best spurge control at 90 DAT. Judge and Neal (2000) also found that reducing Gallery from 1 lb ai/A (1X) to 0.5 lb ai /A (1/2 X), resulted in a drop from 100 percent to 54 percent in spurge control, respectively. They also found Gallery had greater activity in a sand-only media compared to a bark+sand (7:1 v/v) media. Fare and Robinson (2001) also found that containers receiving cyclic irrigation at 45 and 90 DAT versus once daily had significantly less control.

Altland (Oregon) found BroadStar (flumioxazin) and Rout (oxyfluorfen (Goal) + Oryzalin (Surflan), provided excellent to very good control, respectively. More recent studies with Gilliam (Alabama) have found the new herbicide Marengo SC or G (indaziflam) and Biathlon provide exceptional to good control, respectively of prostrate/ spotted spurge. In Neal’s 2007 studies (North Carolina) FreeHand at 150 lb/ac provided superior spurge control versus Snapshot at 200 lb/ac or OH2 at 100 lb/ac and similar control to BroadStar at 150 lb/ac (flumioxazin).
 

Marestail

Even though marestail (Conyza canadensis) is a very difficult weed to control due to its developed resistance to glyphosate and ALS herbicides, I do not assign the difficult strategy to Conyza. The key reason why marestail is difficult to control does not lie in its resistance so much as in lack of understanding that this weed has two life cycles and thus needs to be controlled at two times.

So in keeping with the “weed seed continuum” strategy, marestail can follow a winter annual or a summer annual life cycle, emerging in the fall and in the spring. Fall emerging marestail will have a more extensive root system than those that emerge in the spring. The more established root system of the fall emerging plants make them more difficult to control because they can re-sprout from meristems in the lower part of the stem and roots. Therefore, systemic herbicides are required in high enough quantities to inhibit this re-sprouting. Of course any use of post-emergence herbicides in containers is prohibitive and thus use of pre-emergence controls are preferred.

If larger, older plants are present in surrounding areas or in the container yard, these will have more active meristems and herbicide translocation and early control will be key factors. In fact, glyphosate products provide fairly good control of seedlings 4 inches or less in height where non-resistant populations exist. Many control failures with glyphosate products have occurred when marestail that is greater than 1 foot are sprayed. Weather conditions will also influence the action of the systemic herbicides.

If the fall is relatively dry, fewer seedlings will emerge as winter annuals. If the winter is harsh, fewer weeds will emerge as winter annuals. This will also be true of henbit and chickweed. If the spring is dry, fewer marestail seedlings will emerge as summer annuals. Lontrel is registered for post-emergence control. Pre-emergence products registered for marestail include Simazine, Dimension, Diuron, Gallery, Goal, Oryzalin and Snapshot. In 2012, Randy Zondag (Ohio State University) found excellent control of marestail with Marengo SC applied in fall at 14.5 fl oz/ac that carried through into June, 2013. Marengo seems to be a promising product for those having difficulty with control.
 

Northern willowherb

As a member of the primrose family, Northern willowherb (NWH) (Epilobium ciliatum) is easily recognized by its four alternating sepals and petals when viewed from the top. NWH is also characterized by erect stems that can be one to six feet tall, in a loose clump from a basal rosette of leaves. The leaves are also lance-shaped, toothed and conspicuously deeply veined with short petioles. Foliage and stems are green to purple. A perennial native to moist meadows, stream-banks and roadsides, NWH is another good example of the “we like it here” strategy. NWH takes full advantage of the container environment of luxury water and feeding. It quickly establishes, taking only eight weeks to produce a mature perennial plant from seed. NWH will flower all spring and summer maximizing on its “sheer numbers” strategy. NWH, however, is a widely variable species with several similar subspecies such as E. watsonii, subsp. Glandulosum. Capsule number, number of seeds in a capsule and percent viability of seed within the capsule are also highly variable. The variability is mainly controlled by nutrient loading. In high fertility, a large NWH plant will grow and produce approximately 200 seed capsules/plant with 81 seed/capsule and 65 percent seed viable or 10,410 viable seed. In a low fertility environment, however, as few as 400 viable seed from each plant may be produced. Each seed can germinate within two to four days after falling from the capsule regardless of fertility levels.

The best control of NWH was found to be with granular Ronstar at 200 lb/ac. Sprayed pre-emergence herbicides with Gallery alone or tank mixed provided little to no control of NWH.
 

Liverwort

Marchantia polymorpha L. (a thalloid liverwort) is a common plant pest in nursery and greenhouse production systems. The presence of liverwort is considered unsightly and is often taken as an indication of reduced quality or plant vigor, all of which impacts the final valuation of the crop. The rapid growth and dissemination of liverwort has resulted in heavy thallus mats on the surface of pots, restricting water penetration, competing for nutrients, and providing habitat for other pests and disease vectors. To date there are no registered products that are used by nursery growers for effective liverwort control in enclosed structures. What is reported below are Ohio State University research trials of products that are not registered. We have evaluated SureGuard at ¼ the normal rate in an attempt to reduce phytotoxicity but maintain liverwort control. We have also examined baking soda and MilStop.

We have identified SureGuard at 3 oz./ac (1/4 normal rate); WeedPharm (20 percent acetic acid) at 10 percent v/v, MilStop (5 g/ ft²) and baking soda applied as a dusting (2.24 g/ ft²) can all be effective in controlling liverwort. MilStop in its granule form has an inhalation hazard and is NOT labeled to be applied in this form. WeedPharm will cause phytotoxicity as will SureGuard if not applied dormant. MilStop is an OMRI listed broad spectrum fungicide (with no registration as an herbicide). Used as a spray, MilStop was non-effective for liverwort control. Baking soda is not registered for moss control. However, applications made with a handheld crop duster were very efficacious with no phytotoxicity noted. Further work with rates of MilStop and baking soda are warranted.

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

All photos by Hannah Mathers

In part two of this article, Mathers will continue with creeping oxalis, annual bluegrass, pearlwort, common groundsel and common chickweed.