Using growing media to control weeds

Substrate stratification is a process of layering multiple media (or substrates) in a single container to increase efficiency in container production. An example would be filling a container first with a coarser substrate in the container bottom to improve drainage followed by a fine textured substrate (coir, peat, fine particle bark, etc.) on top to retain more water and nutrients for small liner development. Stratification was first proposed by J. Owen (USDA-ARS), J. Fields (LSU) and J. Altland as a way to increase efficiency in irrigation and fertilization practices. With a fine textured substrate on top of a coarse substrate, water is more evenly distributed throughout the container, improving water use efficiency while simultaneously resulting in less nutrient loss through leaching. Early results in research and on farm trials have shown faster establishment of common ornamentals with 15% less fertilizer and water. (See “Defying Gravity: Utilizing Stratified Growing Media to Invent the Future” in the March issue of Nursery Management: https://bit.ly/stratify-media). As stratification or engineering substrates allows growers to beneficially manipulate water and nutrients, it could potentially be used in other ways to solve different production problems such as weed management, which is the focus of this article.

Stratification for weed suppression

First, a more coarse media will hold less water in the top portion of the container. All things being equal, a ½-inch bark will retain less water than a ¼-inch bark. Many of the most common nursery weed species such as bittercress (Cardamine spp.), eclipta (Eclipta prostrata), and pearlwort (Sagina procumbens) have small seed that must germinate on or near the substrate surface. Furthermore, these species often require high levels of soil moisture to germinate and successfully establish in the container. Having a coarse substrate as the top few inches of the container profile is often enough to reduce germination or significantly slow their growth, reducing hand weeding expenses and delaying seed production. Secondly, the most significant benefit that stratification has provided in research trials related to weed management is that it can act as a form of strategic fertilizer placement. When stratifying for weed control, the top portion of the substrate contains no fertilizer which allows the ornamental liner roots (which are placed in contact with the lower, fertilized substrate layer) to have access to nutrients while simultaneously excluding nutrients from germinating weed seedlings at or near the substrate surface. When used alone, strategic fertilizer placement does not necessarily reduce weed germination, but placement methods such as subdressing at a depth of just 2 inches can reduce growth and vigor of common weeds such as crabgrass (Digitaria sanguinalis), spotted spurge (Euphorbia maculata) and eclipta by up to 90% (Figure 1). Stratification would be a way to use these two different tactics at the same time, reducing weeds’ access to water using coarse substrates, and nutrients via the unfertilized substrate layer, to reduce their overall growth and spread.

Stratification vs. mulching

Using a coarse textured substrate on top of a standard growing mix sounds a lot like mulch, which is currently the most popular and consistently effective non-chemical way to reduce weed growth in container production. Mulching provides similar benefits to stratification in that it holds less moisture than the growing media and dries quickly, reducing weed germination, and it contains little to no plant available nutrients. Mulch also provides a physical barrier to weed germination. Mulching containers with rice hulls, pine bark nuggets, wood chips, and other materials has been shown to be an effective way to reduce weed growth.

There are a few key differences however when it comes to stratification vs. mulch. After a plant liner is potted, mulch is added on the surface and is not part of the rooting volume of a container (Figure 2). Consequently, mulching usually requires that growers leave at least a 0.5 to 1 inch space at the top of the container to make room for mulch application, or possibly wait several weeks or months until the soil settles to apply. In a stratified substrate, that top layer of coarse bark is part of the growing media itself and the layers are within the normal rooting volume of the pot. The top portion of the stratified substrate is coarse, but not so coarse that it prevents development of ornamental plant roots. This increases potential rooting volume and helps to minimize mulch loss due to wind or after pots are blown over during storms. Stratification could also potentially reduce labor costs since it is accomplished at potting, eliminating the need to mulch pots after potting or when they are already on the ground which can be labor intensive. Stratification might seem impractical, and it could be depending on a nursery set up and their potting process. But a lot of growers already pot using a two-part process to increase efficiency and make sure liners are placed at the correct height. Some growers have two soil hoppers or they pre-fill containers and then top them off after placing the liner to make sure the liner is at the correct height. Simply changing the mix that is added in the top portion of the container around the base of the liner to a coarser substrate without fertilizer would allow stratification to take place with no other changes.

Research to date

We have been investigating different methods of stratification and fertilizer placement over the last several years to try and find alternative methods of weed management for container growers. Preemergence herbicides have their place and most likely always will due to their effectiveness and low price point compared to other strategies. However, many herbaceous perennials, succulents, tropicals, ferns, and even common woody shrubs (e.g. hydrangea) are notoriously sensitive. Furthermore, no preemergence herbicides are labeled for use in enclosed greenhouse and hoophouse structures. Reducing hand weeding time in these crops by even 20% or 30% could provide huge labor savings.

In our initial experiments, we used ½-inch and ¾-inch screens for our pine bark to make our top coarse substrates. These substrates contained all particles that were less than or equal to these two screen sizes and were applied at two different depths, either 2 inches or 3 inches. Our bottom “standard” substrate was screened similarly but to 3/8 inches. A controlled release fertilizer was incorporated in the bottom substrate at the same rate in each treatment. We evaluated growth and spread of bittercress (by sowing seeds) and liverwort (by using inoculum pots filled with sporulating liverwort placed around each treatment pot) in these treatments as well as the growth of Ligustrum japonicum and Plumbago aurculata. After 12 weeks, all of our stratification treatments resulted in decreased bittercress growth, ranging from 80% to 97% (Figure 3). The results for liverwort, which loves moisture and nitrogen, where more dramatic where we saw a decrease in liverwort coverage by up to 99% (Figure 4). Growth of both ornamentals was initially about 10% to 20% lower at early evaluation dates (2 or 3 months after potting), but no growth differences were observed at the conclusion of the study at 6 months after potting.

We have trialed many other particle sizes as the top substrate, for example, removing all fines rendering a top substrate that ranges from a ¼ inches to 3/8 inches and applied different top substrates at various depths on other weed species and ornamentals. Overall, the weed control benefit has been largely consistent when we apply a top unfertilized layer of at least 1 inch, we are observing an approximate 40% or more reduction in weed germination and growth on weeds that are not as sensitive to this method (e.g. spurge) and up to 95% or more reduction in weeds that are very sensitive, such as liverwort. More ornamentals need to be evaluated in different container sizes, but to date, if we apply the top layer at a depth that allows the liner rootball to be in contact with the bottom fertilized substrate (dependent on liner size), we have not seen any difference in final growth (shoots or roots) or marketability.

Similar results have been observed in Ohio research using bittercress (C. flexuosa) and oxalis (Oxalis corniculata) as the weed species. Ohio research also showed decreasing weed germination and growth with both increasing the particle size of the top layer, as well as increasing the depth of fertilizer placement in the container. Bittercress and oxalis growth were reduced 80% to 90% when the top layer of pine bark was screened to ¼ inches and ½ inches, respectively. Weed growth was reduced 88% to 99% when fertilizer placement was 2 inches or 3 inches below the substrate surface.

Ohio research also focused on the impact of irrigation frequency. As mentioned above, drying of the substrate surface is a critical component of reducing weed growth. We evaluated pine bark particle size and fertilizer placement in containers that were either irrigated once per day or irrigated with the same volume of water split into three equal irrigation applications throughout the day (using fixed overhead sprinklers). Irrigating with three smaller irrigation cycles throughout the day, also known as cyclic irrigation, kept the container surface wetter for longer periods of time. Applying water in a single application vs. three applications reduced bittercress and oxalis growth by more than 93% across both experiments. Cyclic irrigation has been shown to be an effective tool to increase growth of containerized plants by reducing rootball temperatures on hot days (and perhaps other mechanisms), however, it will also result in greater weed growth if the substrate surface is kept wet throughout the day. One compromise might be to implement the cyclic irrigation but only in a limited period of time during the day (in predawn hours, for example).

Successful weed germination and establishment on the container surface is a function of moisture and nutrient availability. As the experiments described above demonstrate, growing a container crop while keeping the substrate surface dry and free of nutrients (a challenge, to be sure) will greatly reduce weed growth. Substrate stratification provides a new set of tools to manage containers strategically to improve crop growth and reduce weed vigor simultaneously.

Future and ongoing research

Stratification potentially could be a promising part of an overall integrated weed management strategy, but it is still a new area of research and questions remain. For example, in larger containers that can take 12+ months to finish, supplemental fertilization such as topdressing later in the production cycle will most likely be needed to finish the crop. Research is now ongoing to determine what impact this topdressing or other supplemental fertilization would have on any weed management benefit that stratification provides in the early months after potting. We have also primarily looked at small seeded broadleaf weeds and liverwort and have not yet examined at how effective stratification could be on other weed species. Additionally, the original stratification method was proposed as a method to reduce irrigation and fertilization inputs and is implemented inversely to this weed stratification method, and we do not yet know how what effect these weed stratification techniques will have on these inputs. We will continue to investigate this strategy in research experiments and on-farm trials to answer these and other questions to optimize the system and give growers another tool in their weed control toolbox. If you are interested in trialing any of these strategies on your farm, please contact us to learn more about emerging opportunities.

Chris Marble (marblesc@ufl.edu) is an associate professor at the UF/IFAS Mid-Florida Research and Education Center in Apopka, FL. James Altland (james.altland@usda.gov) is the research leader at the USDA-ARS Application Technology Research Unit in Wooster, OH. Research was funded in part by the Florida Department of Agriculture and Consumer Services, Oregon Department of Agriculture, USDA-ARS, Bailey Nurseries and Woodburn Nursery.