Container Weed Management Dr. James Altland
Container vs. field production Containers No seed bank Bark substrates are free of weeds initially Isolated substrate volume Irrigated daily No postemergence herbicides No cultivation Field crops Abundant seed bank Abundant weed seeds and perennial propagules Soil is continuous Irrigated weekly or less Many postemergence herbicides (directed apps.) Cultivation equipment is common and efficient Note the primary differences between weed management in container crops and field crops.
Challenges Weed control must be preventative No postemergence herbicides No mechanized cultivation devices Irrigation is difficult Rates are high Uniformity and precise application rates are difficult High rates of fertility favors weed growth There are challenges to container weed control. Weed control must be preventative. There are no curative treatments, only the very expensive remedy of hand-weeding is a solution. High fertility and non-uniform irrigation also makes weed control difficult. If these two facets of container management can be properly manipulated, overall weed management will be improved.
Opportunities Substrates are initially weed free Substrates are low in nitrogen Unique container environment only allows for a few select weed species Substrates are initially weed free. Contrary to popular belief, weed seeds do not come in with the bark supply, nor do they come with peat moss, sand, pumice, or most other potting substrates. The only exception to this might be compost. Depending on how the compost is handled it may contain weed seeds. Substrates are low in available N. As discussed later, low N levels will greatly reduce weed germination.
From left to right and top to bottom: Creeping woodsorrel (Oxalis corniculata), liverwort (Marchantia polymorpha), pearlwort (Sagina procumbens), common groundsel (Senecio vulgaris), northern willowherb (Epilobium ciliatum), bittercress (Cardamine oligosperma), and prostrate spurge (Chamaesyce maculata) are the most common weed species in Oregon containers. These seven species comprise over 95% of all weeds in containers (in Oregon). Few plants are adapted to the unique container environment.
Container weeds Seed have some sort of active dispersal mechanism Wind-blown Explosive dehiscence Splashed with water Germinate in low light levels Germinate in moist environments Short life cycles Prolific seed producers These are common characteristics of weeds in containers. It should also be noted (but I ran out of room on the slide) that seeds of container weeds are very small.
Container weed management Understand weed biology Understand how weeds propagate and spread Use unique aspects of the container environment to disrupt weed establishment Understand weed biology, then try to imagine how we manage nursery containers can be used to reduce weed growth. This is the crux of the following slide show.
Total weed management program Effective weed control Sanitation Weed seed Weeds in noncropland areas Herbicide management Maintain chemical barrier First I want to talk about the importance of sanitation in weed control. Effective weed management requires a combination of sound sanitation and effective herbicide management.
The pink layer represents the chemical barrier created by the preemergence herbicide. As weed seeds germinate and grow through the chemical barrier, they are either stunted or killed. A uniform chemical barrier is very effective in preventing weed establishment..initially.
The herbicide begins degraded very soon after application. In just days, the barrier already has gaps in which weeds can successfully germinate. The more weed seeds in your production site, the greater the probability they will find a gap in the chemical barrier and successfully establish.
As more time passes, more gaps in the chemical barrier are created. The more weed seed in the container system, the higher the probability seeds will find a gap to successfully germinate.
Weeds growing in pathways and along the roadways of container nurseries increase the number of weed seed in the production system and thus the probability that weeds will germinate in containers.
Again, sanitation in and around the container production site will reduce the number of weed seeds. Notice the photo in the bottom right. Those are bittercress seedlings that germinated and are growing on a single layer of bark that was spilled from a container. These weeds don t require much. Sweeping and removing debris from the production site is an important component to the overall sanitation effort.
Bark is generally free of weed seeds unless you allow weeds to grow in the bark pile!
Sanitation No seed bank in containers Containers placed on gravel Complete vegetation suppression No erosion concerns We can prevent the occurrence of weed seeds in containers Again, our bark substrates and added components are generally weed free. In addition, because these container crops are largely grown on a gravel base, we can completely suppress vegetation (weeds) growing around the production site. Due to the gravel, there are no concerns with erosion.
Because gravel and weed fabrics are used in container nurseries, complete vegetation suppression is the non-crop areas is not only desirable, but also very feasible. There are no concerns of erosion in this situation.
Sanitation Weed control in propagation houses is difficult Many weed infestations come from liners!!!! Pearlwort Creeping woodsorrel Weed control in propagation is difficult, and I believe liners coming from propagation is a major avenue for the introduction of weeds into containers. Invest time into weed control efforts in the propagation phase. Propagation houses are small relative to the potted containers. The liners are in a very high density production site, and it is easier to remove weeds here than remove them after they have reproduced and spread out in the potted containers.
A plot similar to this one was placed at 22 different nurseries in Oregon to evaluate the effects of water quality on plant growth and nutrition. All the liners were donated from high quality nurseries and potted at our research station in early March. All liners appeared healthy and weed-free at the time of potting. However, creeping woodsorrel grew in pots containing euonymus at every nursery, and only in the euonymus pots. Despite our observation of the liners being weed free, the only explanation for this is that the creeping woodsorrel came from the liners, not from the bark, water, or surrounding nursery area.
Observe the deep penetrating root system of creeping woodsorrel. Even if it is removed from the container surface, roots and stolons can readily re-establish the plant.
Cultural practices Fertilization Irrigation Substrate selection
Cultural practices - fertilization Seed of container weeds are small. Seeds must germinate close to the container surface. Seed require available nitrogen (N), phosphorus (P), and potassium (K). Bark contains relatively high levels of P and K. However, bark and most amended substrates (except for compost) contain little or no available N.
Cultural practices - fertilization Seed deprived of N, P, or K will fail to germinate, or germinate poorly. Bittercress seedlings germinated on the container surface, but this photo was taken 3 months after germination. Without available N on the container surface, the weeds could not grow beyond the cotyledon stage.
Pots in the highlighted plot received all fertilizers except nitrogen. Compare weed growth in those pots to weed growth in other pots that received 150 ppm N injected through the irrigation system.
Topdressing sawdust to the container surface is a common practice in Oregon, and this controls weeds very well for about 3 or 4 months. I believe sawdust controls weeds primarily by tying-up nitrogen on the container surface.
Fertilizer placement Container growers use 1 of 3 fertilizer placements for CRFs Topdress, dibble, incorporated
Fertilizer placement Notice that when fertilizers were topdressed or incorporated, weed control generally improved with increasing herbicide (Snapshot) rate. However, when fertilizers were dibbled, very few weeds grew in any pot regardless of the herbicide rate.
From left to right, observe weed growth and lavender growth in containers that where fertilizers were topdressed, incorporated, and dibbled.
Guidelines for dibbling Trial on each crop first Use only homogenous blends of CRFs Use only CRFs with release of 8-9 month or longer Only use when potting in spring or early summer Do not use when potting in fall.
Cultural practices - Irrigation Over-watering crops decreases weed control. Increased moisture increases chances of seed germination and survival. Excess water increases microbial activity, and microbes degrade herbicides.
Observe weeds growing only where irrigation wets the container surface.
Efficient irrigation Requires Precise application rates How much water should you apply? When do you apply it? Uniform irrigation This is a major limiting factor to efficient irrigation
Uniform irrigation Easy with pressure compensating microemitters Large containers Difficult with overhead irrigation Without uniformity, efficient overhead irrigation is unlikely
The Gantry irrigation system commonly used in Holland and Germany applies water more uniformly and thus more efficiently than overhead irrigation sprinklers commonly used in the U.S.
Substrate selection Select a media that will dry quickly on the container surface. When container surface dries quickly: Weeds grow poorly. Liverwort suffers. Herbicides persist longer.
Pearlwort Substrate particle size Seed are very small. When coarse substrate is used, seeds fall too deep in pores, cannot germinate. From left to right, pearlwort germination in pots with increasing substrate particle size.
Mulch products Disks Plastic weed lid Coco disk Geotextile disk Loose fill Sawdust Biotop Bark chips There are two major classifications of mulches used for weed control in container production disks and loose fill mulches.
Coco disk Made from coconut husks Similar to other disks More forgiving to plants without single stems Excellent weed control $0.09 / #1 container Coco disks have become a favorite of many Oregon nurserymen.
More photos of the coco disk.
Sawdust Cheap Readily available Provides control for several months Easy to apply Removes N from container surface Starves weeds on container surface Less than $0.01 / #1 container
Photos of sawdust being used, and on the bottom is a photo of a machine used to apply loose fill mulches (sawdust) to containers. The photo was taken at a nursery in Canada.
Container mulch characteristics Coarse Hold little water Dries quickly after irrigation Little or no available N Avoid compost Resist decomposition Locally available and cheap
Summary Herbicides alone will only be marginally effective Sanitation is your most powerful tool Understand weed biology (there aren t many) Use sanitation and cultural practices to limit success of weed spread and growth
Website http://oregonstate.edu/dept/nursery-weeds/