Cultural And Sanitation Practices

James Altland, Ph.D.
North Willamette Research and Extension Center
Oregon State University

Introduction

Weed control in container production must be a preventative effort.  A successful weed management program will prevent weed germination and growth by a combination of sound cultural and sanitation practices along with a proven effective preemergence herbicide (or perhaps a non-chemical mulch).  If you rely solely on herbicides, you will fail.  Good cultural and sanitation practices will reduce the number of weed seed infesting your containers, thus reducing weed pressure and making herbicides more effective at preventing weed germination and development.  

Maintaining the Herbicidal Barrier

container
Herbicides form a chemical barrier over the container media.  Though each herbicide controls weeds differently, they all provide control at the point which germinating seed emerge through the chemical barrier.  If the chemical barrier is disrupted, it will create a gap where weed seed can successfully germinate and grow.  There are several common practices that disrupt the chemical barrier, these include but are not limited to: poking holes in the barrier with your fingers or hands while moving containers around, dropping containers, and containers blowing over.  All these activities should be minimized to prevent disruption of the chemical barrier.  Instruction and explanation of this concept to the work crew is necessary, as they are typically the ones responsible for moving and working around the containers.

Pulling uncontrolled weeds will also create gaps in the chemical barrier.  Weeds should be pulled before they go to seed, however, soon after removing weeds from an area, a follow up herbicide application should be made to generate a new chemical barrier and prevent germination of more weeds.  Be certain to follow label precautions on intervals for reapplying herbicides.

Sufficient and Uniform Herbicide Application

To create an effective chemical barrier over the container media surface, herbicides should always be applied at the rate specified on the label.  If rates are too low, the chemical barrier may not be sufficient to prevent weed growth, and if rates are too high the herbicide may cause crop injury.  Also, it is important to apply herbicides uniformly.  When herbicides are not applied uniformly, weed seed will germinate in areas with insufficient herbicide.  Use properly calibrated equipment that is functioning correctly.  Even when this is done, flaws in equipment engineering may still result in non-uniform applications.  Research at North Carolina State University (Darden and Neal, 1999) demonstrated that even with trained staff using properly calibrated equipment, the actual amount of applied herbicide varied throughout the treated area as low as 0.5 and as high as 2.2 times the intended rate.  Meticulous attention should be given to equipment calibration and application uniformity, and steps should be taken to ensure proper herbicide rates are applied.  Monitoring uniformity with collection pans placed throughout the application area can be used to measure uniformity of application.  This would be similar to using rain gauges to measure irrigation uniformity.  Identifying areas with non-uniform applications will help prevent regions with weak chemical barriers.  

Production Practices

Shade, fertilization practices, irrigation, and media affect weed populations.  Research with liverworts (Marchantia polymorpha) (Svenson, 1998; Svenson et al., 2001) demonstrated that they thrive in moist environments with high levels of available nitrogen and phosphorus.  Thus any practice that allows the container media surface to dry quickly, or removes nitrogen and phosphorus from the container surface, results in improved liverwort control.  Cultural practices that reduce liverwort infestations include improved air circulation, use of a mulch, use of coarser container media, and dibbling or incorporating fertilizer instead of topdressing.  Herbicides such as oxadiazon and oryzalin alone failed to provide adequate liverwort control in Oregon State University research.  Overall, it was concluded that a combination of herbicides and cultural practices was necessary for optimum liverwort control.  

Current research at the North Willamette Research and Extension Center is evaluating how fertilizer placement affects weed growth.  Our research has demonstrated that dibbling fertilizers reduces growth of oxalis, common groundsel, and other weeds compared to when feritlizers are topdressed or incorporated.  Dibbling fertilizers generally improves growth over incorporating, and provides similar growth to topdressing.  

Feritlizer manufacturers generally recommend against dibbling fertilizers due to liability concerns.  If you consider dibbling feritlizers, try the method on 50 to 100 plants of each species before committing the method your entire stock.  Only use products that are rated as 8 to 9 month formulations or longer.  There may also be problems when plants are potted (and dibbled) late in the fall, due to potential salt buildup over the winter.

Sanitation: Practices For Excluding Weed Seed from Containers

Practices that minimize the number of weed seed in the production system will improve weed control.  The more weed seed that is allowed to contaminate containers, the higher the probability weeds will germinate in areas with a weakened chemical barrier. 

Weed control under containers

Containers should be placed over some sort of groundcover, usually stone or weed-fabric.  While this method is effective in suppressing weeds under the covered area, debris from plants and spilled bark will create an environment where weeds can establish.  Weeds between containers are a source of weed seed (bittercress and oxalis can project seed several feet).  Other weeds like eclipta can become established in the drainage holes and out-compete the plant for water and nutrients.  So even the area under containers must be maintained weed free.  

eclipta under containers

Between crops when beds are empty, existing weeds should be removed or chemically controlled, fresh stone or new weed fabric installed if necessary, and the area should be swept and made debris free.  Herbicides can be applied directly on the stone or weed fabric.  Barricade 4L or 65WG (prodiamine) is an excellent choice for this application due to its low solubility (0.013 ppm).  Barricade is labeled for this use, and research has shown it provides weed control in gravel beds for up to 8 months (Briggs et al., 1998).  Other preemergence herbicides are also suitable for this application, but only those with low solubility (less than 1 ppm) should be used.

Weed control in non-cropland areas

weeds beds
Eliminating weeds in non-production areas such as roadways, drainage ditches, between hoop houses, etc., will drastically reduce weed seed number and improve weed control.  This can be accomplished with regular mowing of turf areas as long as it’s done often enough to prevent turf weeds from setting seed.   Mechanical removal such as disking or plowing can be used, though this makes the area more susceptible to erosion and washouts. 

Control with herbicides provides effective control.  Postemergence herbicides can be used to eliminate existing weeds, and preemergence herbicides used to prevent re-growth.  Maintaining weed-free non-cropland areas is probably the easiest and the most effective sanitary practice for reducing weed seed numbers in your containers.

Weed control in bark piles

Bark piles should be kept weed free.  Not only will these weeds generate seed that can be blown into containers, but they can also deposit seed and/or vegetative propagules (tubers from nutsedge, rhizomes from oxalis, etc.) directly into the media that will eventually be used for potting.  When bark piles are kept weed-free, they do not contribute as a source of weeds (Cross and Skroch, 1992).  Steam pasteurization, solarization, composting, and fumigation are some treatments that will kill seed and other propagules in bark piles.  These treatments are not likely cost effective for most nursery operations, and simple sanitation will avoid the need for these treatments.  

bark pile

Clean pots

bc liners
Use of clean or new pots for propagation and/or canning will also reduce the number of weeds in your nursery.  A study at Clemson University (Bachman and Whitwell, 1995) demonstrated that by simply washing propagation pots with pressurized water, the number of germinated weeds was reduced six-fold.  Their study showed that when dirty pots with debris around the edges were used, weeds (bittercress) germinated around the edges of the pots, compared to few weeds when new or cleaned pots were used.

Wind-breaks

Even if you can effectively control weeds throughout your nursery, it might be impossible to control weeds growing on neighboring property, yet these plants can also disseminate seed.  Use of a wind-break, such as a tall hedgerow or fence, to minimize the amount of seed entering the nursery will improve weed control.  The hedgerow or fence need not be permanent.  One local nursery lined a side of their property (bordering a weedy lot) with large 15 gallon plant material (mostly trees and large shrubs).  This solution works doubly well by providing additional production area and blocking weed seed from entering the nursery.

Start with weed free liners

bc Use of weed free liners is critical, especially when dealing with weeds that have extensive root systems such as oxalis, liverworts (rhizoids instead of roots), and pearlwort.  Because roots from these plants can generate new plants, thorough hand-weeding to remove shoots and roots is necessary.  A single escape weed can generate new plantlets at an alarming rate.  One bittercress plant can produce up to 5000 seed in just 5 weeks (Bachman and Whitwell, 1995).  

In conclusion, weed control in container production must be a preventative effort.  Use of herbicides alone will result in failure.  The most effective weed management is provided by using sanitation to reduce weed seed numbers in your productions system, while maintaining an effective chemical herbicide barrier.  If herbicide use is over-emphasized while neglecting sanitation, increased numbers of weed seed will likely find someplace to germinate where the chemical barrier has been weakened.  On the other hand, if sanitation is made a priority while herbicide applications are neglected or implemented improperly, the few weed seed that are present will likely germinate and compound the problem.  Cultural and sanitation practices are the beginning to a successful weed management program.

Literature Cited