Mulches for weed control in containers

Text by James Altland, photos provided by the Canadian Nursery Landscape Association and the British Columbia Ministry of Agriculture

Introduction

Preemergence herbicides are an integral part of most weed management programs; however, they are not always practical. Preemergence herbicides are not labeled for use (and therefore not legal) inside enclosed structures such as greenhouses, hoop-houses, and other covered structures. Some crops are sensitive to preemergence herbicides, most notably hydrangeas and many herbaceous plants.

When herbicides are not an option, there are alternatives to chemical weed control. This article will cover some of the non-chemical alternatives available to nursery growers. We discuss the advantages, disadvantages, and material costs for each product. Labor costs for alternative weed control are difficult to quantify and predict. Nonetheless, this article should provide a starting point for those seeking an alternative to herbicides.

Mulch types

There are two broad categories of container mulches. The first group are ‘disks’ that cover the container surface and form a physical barrier through which weeds cannot penetrate. Examples include the geotextile disks, coco fiber disks, and plastic weed disks. To varying degrees, these products are rigid, so container size and placement of the plant in the container will affect how well the disk ‘fits’ on the container surface. Often, these products leave gaps around the edge and around the hole cut out for the plant stem. Weeds exploit this gap readily. This ‘lack of fit’ is the primary problem with these products.

The other group of products includes ‘loose-fill’ mulches. These products form a layer over the container surface through which weeds cannot germinate due to physical or chemical characteristics of the mulch. Most are byproducts of some other agricultural operation which makes their cost affordable, but their availability sometimes seasonable.

Loose-fill mulches that consistently perform well share similar qualities. The most successful products should have a combination of the following properties: they provide little or no available nutrients, they dry quickly after irrigation, they resist decomposition, they are easy to apply, cost effective, and are commercially available. The final criteria for using any alternative product is its acceptance by end-users.

The following is a short-list of alternative products with potential in Pacific Northwest container nurseries.


Geotextile disks

Geotextile disks are gray cloth-like disks. They are permeable to water and nutrients, but form an impenetrable barrier that prevents weeds from growing up through.

The advantage of using this product is that only one application is necessary for the life of the crop. Some nurseries have reported re-using the disks at least once. The disks are light-weight, easy to install, and easy to remove at shipping.

The disadvantages of this product are that they generally work only for crops with a single stem. Also, the trunk must be in the center of the pot or they fit poorly. Weeds germinate around the edge of the disk and from the hole cut out for the stem. The biggest deterrent in using this product has been their propensity to blow away. This can be remedied by using florist greening pins (1 ¾”) to hold the disks onto the container surface. It is thought that florist pins rust quickly in the bark thus anchoring itself and the disk on the container surface.


Coco-disks

Coco-disks are made from shredded coconut husks. They are formed in a disk with a shape similar to geotextile disks. The disks are approximately ¼ inch thick. According to ongoing research in Canada, they provide excellent weed control. Similar to geotextile disks, they are designed for plants with a single stem positioned in the center of the pot. However, they reportedly accommodate multi-stem plants and plants that are not perfectly centered, more so than geotextile disks.


Plastic weed lids

The plastic weed disk shown in Figure 3 is the latest in an evolution of similar plastic products. Because it snaps over the rim of the container, and essentially 'locks' into place, weed control around the edge of the container is better than other disk-type products. However, as shown in the image to the right, weeds might still grow up around the center hole. Because the plastic is relatively rigid, some nurseries have commented that it rubs against plant stems and wounds thin barked plants.

Liverwort (Marchantia polymorpha) and pearlwort (Sagina procumbens) are two of Oregon’s most problematic weeds. It has been reported that placing these disks on containers already infested with these weeds will kill them. Both weeds are prostrate growing, so this might be reasonable to expect. However, it probably will not work for upright growing weeds such as bittercress (Cardamine hirsuta) or common groundsel (Senecio vulgaris).

Another potential benefit of using this product is that these disks should be durable enough to use for several consecutive crops, thus gradually reducing the 'per container' price listed in Table 1. It has also been pointed out by some users, that when the plastic weed lids are installed after fertilizers are topdressed (so the fertilizer is below the lid), containers that blow over do not spill the fertilizer onto the ground.


Hazelnut shells

Hazelnut shells have proven a successful mulch for Oregon container nurseries. OSU research has shown that hazelnut shells alone provide excellent liverwort control for up to 8 weeks (Svenson, 1998). Hazelnut shells in combination with labeled rates of Ronstar (oxadiazon) provided excellent liverwort control for 12 weeks. Additionally, individual nurseries have commented that hazelnut shells are effective in weed control.

Hazelnut shell availability is an issue that needs consideration. There are lots of shells available each year for nursery growers. However, they are only available during the time of year nuts are being processed (mid-October through December). Nut processors I spoke with have no way of storing the shells, so nurseries have to buy them as they are being processed. Hazelnut processors send most of their shells to be burned as a source of energy (they use the term 'hog fuel'). At one operation, they have to dispose of the shells every couple of days, and the only efficient outlet for disposing of shells has been to burn them. Nurseries can send trucks over to the processing plants get shells during the few months of availability. Then it's up to the nursery to store them until needed.

Overall, it’s my impression that hazelnut processors are happy to sell the product to nurseries, but they will not go out of their way to do so. If there were more demand for the product, it might be something that soil and bark distributors could re-wholesale.

Some have speculated that use of hazelnut shells might spread Eastern Filbert Blight (Anisogramma anomala), a disease causing serious problems for Oregon’s hazelnut industry. However, OSU’s plant pathologist, Dr. Jay Pscheidt, says shells will not carry or spread the disease. The only way the disease could be vectored through this product is if twigs from an infected tree are somehow mixed in with the shells.

One of the factors in deciding if a mulch product is feasible in container production is the perception of end-users. Most consumers I’ve talked to agree that use of hazelnut shells is an ‘eco-friendly’ thing to do, with few negative drawbacks. However, I heard from a retail garden center manager once who complained that the round shells are difficult to sweep off concrete floors in their retail area.


Shredded tires

Research at Auburn University (conducted by me as a graduate student) demonstrated that shredded tires provide excellent weed control.

Crumb rubber is produced by mechanically removing the steel radials from tires, then shredding the rubber portions. Large magnets are used in the final grading process to ensure all the metal is removed. Crumb rubber is usually processed into batches of different particle size, from ¼ inch and smaller.

The product is black, uniform in color and particle size, light weight, and relatively dust-free. It is easy to handle and apply. It does not hold water so it dries quickly. Rapid drying after irrigation is the primary reason it provides excellent weed control.

Many nurserymen scoff at the idea of using crumb rubber, assuming the general public or other end-users would not accept the product. That’s an unresolved debate, but certainly a point worth considering.


BioTop

Biotop is a starch/straw-based material. It is a loose-fill mulch with similar texture to sawdust. It has been used by several nurseries in British Columbia and Oregon. The product is produced in Denmark, and used throughout Europe. There is little information about it in the U.S., and most of what I know is from personal conversations with nurserymen that have used it.

A British Columbia nurseryman reported that it does not provide much better control than sawdust; however, it cost about 10 times more. The same nurseryman also reported several crops are sensitive to the product. The crops experienced reduced growth, die-back, and were generally not salable (hydrangea and summer flowering heather).


Sawdust

Sawdust is the cheapest and most abundant byproduct with potential for use as mulch. It is available most times of the year, although some distributors do not like to carry it over the wet winter months.

In order to decompose sawdust, microorganisms must scavenge large quantities of N on the container surface. With no available N on the container surface, weeds often fail to germinate or grow beyond the cotyledon stage. Applying sawdust to the container surface will have no effect on N below the container surface, and thus have no measurable effect on plant growth and performance.


Applying mulches

The major criticism of using mulches in container production is the labor costs to apply them. Disk-type mulches are probably more difficult to apply than loose-fill mulches. Disk-type mulches have to be fit to the container, and some have to be held in place with pins to prevent them from blowing away.

The best method for applying loose-fill mulches that I’ve seen comes from a British Columbia nursery. They use a hopper that dispenses the mulch onto a wagon of recently potted containers (Figure 4). A front-end loader dumps sawdust into the hopper. The entire unit swivels around so a wagon can back up underneath it. A motor on the left side turns a drum inside that dispenses the sawdust in an even layer over the container surface.

Are they economical?

Table 1 provides a list of mulches discussed in this article along with product cost. Most mulches, even the cheap ones, cost more than a year’s supply of herbicide. Labor is the most difficult factor to quantify. How much time will it take to apply the mulches compared to the time it takes to apply herbicides? Some mulches provide excellent weed control for the production life of the crop, with little or no additional hand-weeding. Will these products result in reduced labor? Does reduced labor justify the extra product cost? Answers to these questions are difficult to predict, and will have to be estimated by individual nursery operations.

Conclusion

Currently, some of Oregon’s largest and finest nurseries (in terms of crop quality) are using mulches in some crops. Whether or not mulches are feasible for container production has to be determined by each individual nursery operation. In situations where chemical herbicides are not safe, not labeled, or where crops are not tolerant, mulches described in this article offer an alternative.


Literature cited

Svenson, S.E. 1998. Suppression of liverwort growth in containers using irrigation, mulches, fertilizers, and herbicides. SNA Proceedings 43:396-402.

Preemergence herbicides cannot be used in enclosed structures such as hoophouses and greenhouses.
 
'Disk-type' mulches fit over the container surface, with a hole cut out for the plant stem.
 
Geotextile disks are a fabric material that covers the container surface.
 

Coco-disks (the image above and the previous image) are made from shredded coconut husks.

Plastic weed disks fit snugly over the container lip.
 
Hazelnut shells are cheap and locally available. However, they are only seasonally available and must be purchased in large quantities and stored until needed.
 
Hazelnut shells are ideal for use in greenhouses. Above they are used for weed control in greenhouse-grown ginkgo trees.
 
Crumb rubber is an attractive and effective mulch, but consumer acceptance is questionable.
 
Sawdust is a cheap and readily available mulch product, and is currently used on many herbicide-sensitive crops.
 
Machines have been built to apply sawdust and other mulches efficiently (learn more about this machine).

 


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