An OSU associate professor and weed specialist offers a silver linings playbook to help you control invaders in your container production.
If your nursery container weed control program is working optimally, the only time you should need to hand weed would be one to two days before each herbicide application. Your scheduled applications would be in the spring, summer and fall for most Midwest container nurseries. With longer lasting pre-emergence herbicides, your scheduled applications may only be spring and fall. In regions with longer growing season’s applications, two or three applications may be required over the summer. Sometimes, however, weeds may go to seed before a scheduled application. In this event, supplemental hand weeding would be required followed by a half rate of pre-emergence herbicide. Only a half-rate is required as you are only re-establishing the chemical barrier in the top half-inch of the soilless media.
FreeHand (dimethenamid-p+pendimethalin) applied at twice the normal rate (i.e. 300 lb./ac) to elderberry (Sambucus nigra Blacklace ) spring 2012 caused significant stem brittleness and weakening
The major cost of your container weed control program is labor to hand weed when your pre-emergence herbicides fail. The relative cost of the pre- and post-emergence herbicides is small by comparison to labor. Reducing your hand weeding to the two or three events, with the best herbicides, methods and timings, is essential to saving money. The cost of herbicides, however, also includes labor for the application. The cost of applications can be reduced via new methods and equipment. Applying at the correct time will also reduce your application costs by increasing the efficacy of the application.
FreeHand (dimethenamid-p+pendimethalin) applied at twice the normal rate (i.e. 300 lb./ac) to elderberry (Sambucus nigra Blacklace ) spring 2012 caused callus proliferation at the plant crown.
Photos courtesy of Hannah Mathers
How do pre-emergence herbicides work?
Many of the pre-emergence herbicides used in ornamentals must be applied before weed seeds germinate, as they are inhibitors of microtubule assembly (Weed Science Society of America [WSSA] Group 3 herbicides). The mode of action (MoA) of Group 3 herbicides is mitosis inhibitors (Mi). These Mi herbicides include the dinitroaniline herbicide family, which are root inhibitors and represent our most common herbicide family in the ornamental industry. Barricade 65WG (Prodiamine); Pendulum 2G, Pendulum 3.3 EC, Pre-M 60DG, Pre-M 3.3 EC, or Corral (Pendimethalin); Surflan T/O (Oryzalin); Treflan 5G or Trifluralin EC (Trifluralin) are all dinitroaniline herbicides. Dimension (Dithiopyr) or Dacthal (DCPA) are also Mi herbicides in the pyridine and benzoic acid herbicide families, respectively.
Mi herbicides also include shoot inhibitors such as the chloroacetanilide herbicide family (for example, Pennant [metolachlor] and Tower [dimethamid-p]) and the acetamide family (Devrinol [napropamide]). For Mi herbicides to be effective, the germinating seed must contact the top half-inch of the media surface where the herbicide has been incorporated. If the seed is not in contact with the incorporated layer, germination will not be inhibited. Any seed that has already germinated will be unaffected by the Mi herbicides.
Root absorbed Gallery (isoxaben) a cell wall synthesis inhibitor herbicide caused severe root and shoot stunting to Spiraea japonica ‘Neon Flash’.
During mitosis, identical copies of the genetic material are split and pulled apart so that two daughter cells arise from one mother cell. Spindle fibers, formed in the dividing cell, are the structures that pull the sister chromatids apart. Spindle fibers are composed of microtubules made from tubulin. Dinitroaniline herbicides impact the formation of tubulin. The result is root cells with multiple sets of chromosomes (polyploidy) and faulty cell walls incapable of further growth or nutrient absorption. Symptoms include clubbed or swollen root tips, yellowing or purpling of leaves and stems often associated with nutrient deficiencies. Whitening may also occur due to inability to produce chlorophyll. Weak stems are common due to poor cell wall development. Callus proliferation is another symptom. Callus is usually formed from structural tissue, not individual cells. The disruption of mitosis leads to the proliferation of callus. Injury with Mi herbicides usually occurs when they are applied too frequently or at too high a rate. Species tolerance also plays a significant role.
Symptoms of injury caused by the PPO inhibitor herbicide BroadStar (flumioxazin) such as leaf crinkling and malformation on variegated Liriope
Some non-Mi pre-emergence herbicides, however, will kill “small” weeds post-germination. These exceptions are Goal (oxyfluorfen), SureGuard (flumioxazin) and Ronstar (Oxadiazon), which are Group 14, protoporphyrinogen oxidase (PPO) inhibitors, and Gallery (isoxaben), a Group 21 cell wall synthesis inhibitor (CWSi). PPOs are found in the plant chloroplast and their inhibition leads to lack of a precursor of chlorophyll. PPO inhibitor damage can appear as mottling, leaf crinkling, malformation and puckering from the rapid destruction of the contacted tissue.
Symptoms of injury caused by the PPO inhibitor herbicide BroadStar (flumioxazin) such as rapid destruction of contacted tissue Spiraea species.
Cell walls occur in all plant cells. CWSi herbicides prevent new cells in the root and shoot apices from producing cell walls causing plant growth to cease. When absorbed by roots, Gallery moves primarily in the xylem and causes severe root and shoot stunting. When absorbed by leaves, however, there can be slow basipetal or phloem translocation that results in mottling and random leaf chlorosis and necrosis of contacted tissue.
Damage from Gallery + Barricade at 1.0 lb. + 0.66 lb. ai/ac, respectively on Echinacea ‘Purple Magnus’ at two weeks after application showing severe mottling from leaf absorbed Gallery (isoxaben), a cell wall synthesis inhibitor herbicide.
Another non-Mi herbicide, new in 2013, is Marengo (indaziflam). Marengo is a selective pre-emergence herbicide that offers long-term residual control of both grassy and broadleaf leaves. Indaziflam is the first active ingredient for ornamentals from the Group 29 MoA. Group 29 herbicides inhibit cellulose biosynthesis. From our 2012 Ohio State University trials, we found indaziflam caused no injury applied at normal rates on Buxus ‘Green Velvet’, Rosa ‘Knockout’, Berberis thunbergii ‘Crimson Pygmy’, Itea ‘Little Henry’, Viburnum plicatum ‘St. Keverne’, Viburnum ( ‘Juddi’, Rosa ‘Home Run Red’ or Euonymus ‘Compacta’.
Mottling and random leaf chlorosis caused by the cellulose inhibitor Gallery on Buddleia davidii.
The weed emergence continuum
There are two big times in container nursery weed control based on weed life cycles, namely the emergence of summer and winter annuals. These two life cycles dictate two of the three “scheduled” herbicide applications noted in the introduction.
Bittercress (Cardamine pensylvanica) is a very problematic winter annual weed. The long, thin seed pods or siliques are shown. When dry the siliques are dehiscent fruits that split down the two long sides causing the seeds to “explode” from the two halves of the silique.
Summer annual (SA) weeds germinate in spring and die by winter; these include such plants as lamb’s-quarters (Chenopodium album), common purslane (Portulaca oleracea) or northern willowherb (Epilobium ciliatum). Pre-emergence herbicides need to be applied and incorporated in mid to late March or following the removal of the poly-coverings from the overwintering houses to control the SAs. Winter annual (WA) weeds germinate in fall, overwinter as seedlings and set seed and die in the summer, such as shepherd’s purse (Capsella bursa-pastoris) or Pennsylvania bittercress (Cardamine pensylvanica). Pre-emergence herbicides need to be applied and incorporated by mid to late August or prior to applying the poly coverings to the overwintering houses (September to November) to control WAs.
Marestail (Conyza canadensis) an increasingly problematic nursery weed that has two life cycles summer and winter annual.
As with any continuum, some winter annuals emerge before their counterparts – for example, marestail – and some summer annuals after their counterparts – for example, prostrate or spotted spurge and purslane. To control marestail, herbicides need to be out and incorporated by early August. To control prostrate or spotted spurge and purslane, pre-emergence herbicides need to be applied in June when temperatures are rising to levels adequate to germinate these weed species. Because these late and early emergence weeds are not fitting into the “scheduled” application times, it is understandable that the herbicides that control them best are PPO or CWSi or PPO- or CWSi-containing herbicides. This also explains why the most difficult and prevalent container weeds shown in Table 1 are also controlled by these MoAs.
Other weeds that don’t fit the “scheduled” herbicide applications are those that are dose-responsive, such as crabgrass and bittercress. Products with sufficient residual are required to control these weeds. FreeHand 1.75G, a newer herbicide from BASF, applied at 150 lb/ac is an option for these dose-responsive weeds. FreeHand is proving to be an excellent, broad-spectrum weed control; at 150 days of control, it is one of the longest-lasting pre-emergence herbicides in container nurseries.
Prostrate spurge (Euphorbia maculata) is a difficult summer annual weed in containers and container yards due to its late germination and love of sunbaked conditions.
Because weed emergence occurs as a continuum, combination herbicide products are often superior. Not only do they contain one of the three “burn-down” potential herbicides for seeds already germinated at time of application, but they also usually contain grass and broadleaf active herbicides.
Five difficult and prevalent container weeds and controls determined for each.
Liquids versus granule applications
More nurseries are turning to liquid versus granule applications to save money; savings occur as liquid formulations cost less than granules. Liquid applications are also more uniform than granules, which reduces waste. However, most of the significant savings occurs in the reduction of time spent applying liquids versus granules – or the labor savings.
The most common liquid combinations being used are Gallery (1.33 lb/ac) + Surflan (2 qt/ac) or Gallery + Barricade (each at 1 lb/ac). These applications were pioneered by the late Dr. John Ahrens at the Connecticut Experimental Station in Windsor, Conn. For these applications, flood jet nozzles (such as a KLC-5) with a backpack sprayer are used. Flood jet nozzles can spray swaths of 14 feet and should be applied in two directions walking down the house and then back up at a normal walking speed of 3 feet per second.
Most commonly used granular pre-emergence herbicides registered for nursery containers. Note that many are combination products that have selectivity on broadleaf and grasses in order to increase their spectrum and flexibility of control. BroadStar is the only granule listed that control broadleaf and grassy weeds with a single active (flumioxazin).
It is crucial that a pressure regulator or a pressure-regulated backpack sprayer be used to maintain a constant psi of 20 throughout. Conventional belly-grinder applicators are commonly used for granule applications; multiple trips to fill and refill the belly-grinder are laborious and time consuming. Filling the tank of a backpack sprayer from a hose connected to a 100- or 50-gallon sprayer that is constantly agitated, contains the liquid herbicide required and positioned at the end of the house is quick and time efficient. Growers who have switched from hand-cranked granule to liquid backpack applications report reducing 38 percent of their man-hours per acres, per application.
Backpack sprayers can be used to control weeds 3 to 10 feet adjacent, including between the polyhouses.
Control in the container yard?
Using the equipment also simplifies your container yard weed control. The only exception to the procedure would be the use of a flat fan nozzle, such as an 8004 or an 8004 VisiFlo for an even, flat spray instead of the flood jet nozzle. In an area of 3 to 10 feet adjacent to the polyhouses, including between the polyhouses, monthly sprays of glyphosate (Roundup) or Finale (glufosinate-ammonium) may be required. Two or three times in the season, to add residual control, additions of Surflan + Barricade or some other DNA herbicide can be added to tank mix with the glyphosate or glufosinate. In the roadways of the container yard, a four-nozzle, one-direction boom can be constructed to come off a 50- or 100-gallon tank mounted on the back of a tractor. Again, 8004 flat fan nozzles are used, spaced 40 inches apart. Using this system the tractor driver goes up one side of the roadway and back the other at a speed of 3 mph to deliver the same herbicides or herbicide combinations as prescribed for the backpack spryer. Beyond 10 feet from the houses, regular mowing is required. Ensure no weeds go to seed or rhizomes creep from these areas to infest the houses and containers. Geotextile ground fabrics can also be placed under your containers, as a weed free barrier, in the polyhouse.
One-directional spray booms mounted off spray tanks on tractors can be used for liquid herbicide applications in the roadways of the container yard.
Identify the culprit
As a general rule, for every pound of weed growth produced, about one less pound of crop growth is produced. Usually, early weed competition reduces crop growth far greater than late season weed growth. Therefore, early weed control is very important. Late season weed control is important in the container yard and always for reducing the seed bank for coming years.
Despite the “continuum” of weeds, a silver linings playbook can be developed. The playbook as outlined consists of seven plays to control the CULPRIT:
- Check the label for crop tolerances;
- Use combination or single active ingredients that control grass and broadleaf weeds as granules or liquids;
- Liquid combinations or single active ingredients for lucrative “burn-down” or “residual” to control weeds not fitting major weed emergence events;
- Prevention in the areas adjacent to the container houses;
- Right herbicide for the time of application;
- Initiate applications to proceed weed emergence events;
- Ten feet beyond to houses, mow to limit weed seed dispersal
Associate professor Hannah Mathers is State Extension Specialist Nursery/Landscape in the Department of Horticulture and Crop Science at the Ohio State University; she also serves as Chair of the OSU Nursery Short Course. She can be reached at