The most common definition of a weed is a plant or a population of plants growing out of place. These plants are growing where they are not wanted. To properly appreciate this concept, let’s look at it from the point of view of a nursery grower.
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Effects of weeds on crops
Weeds cause bad things to happen to the plants we hope to make money on as we try to keep them healthy. Here are the most common effects weeds have:
* Some weeds produce toxins that can stunt or kill other plants.
* Most weeds are aggressive competitors. They thrive on fewer nutrients. This means they keep going when many crop plants come under stress. They can thrive under a wider range of soil moisture and drainage conditions.
* Under higher nutrient levels, most weeds grow quickly and get quite large. Because of their size, weeds grab space, water and even nutrition from crop plants.
* Weeds often block light from crop plants.
* Weeds can harbor damaging pathogens and pests.
>strong>* Weeds interfere with harvesting of crop plants.
* Weeds reduce the value of crop plants.
* Customers may perceive that poor weed control relates to careless cultural practices such as irrigation techniques, pruning, fertilizing and other pest-control measures.
* Customers do not want to buy plants with weeds in them because that will lead to landscape weed contamination.
* Most weeds reproduce and/or spread rapidly.
Types of weeds
Weeds fall into various broad groups that relate to their management. An understanding of these groups is needed to properly appreciate management practices. My grouping is in outline form to make it easier for you to remember them.
Weeds can be grasses (monocots) or broadleaved (dicots). This is important as it relates to herbicides used in their management.
Broadleaf weeds can be herbaceous or woody. Woody weeds, like woody crop plants, are slower to develop. We do not generally consider them in chapters on weed management. There are exceptions, such as purple loosestrife.
The life cycles of weeds vary. This is important in management. It especially relates to the kinds and timing of herbicides used.
Summer annuals come up from seed each year. The seeds germinate in spring and set seed and die by the coming winter. The seed is the overwintering structure. Lambsquarters is a common summer annual.
Winter annuals also come up from seed each year. The seeds germinate in fall, overwinter as seedlings, grow in early spring, set seed in summer and die. Winter wheat is a winter annual.
Biennials are a bit unusual. They live for two growing seasons. During the first year, they act similar to a summer annual or a winter annual. However, they do not set seed during this first year. They form a rosette or crown of leaves and a root system that can store reserve carbohydrate over the first winter. In the second year, biennials sprout back and produce seed like a summer annual. Western salsify is a biennial weed.
Bulbous perennials live for many years, with underground bulbs as their primary means of reproduction. They can become problems because the bulbs are difficult to kill with most herbicides. Many produce large numbers of bulblets. Wild onions or violets are good examples.
Simple perennials live for many years as well. However, they reproduce mainly by seeds. Offshoots or pieces of the underground portions of the plants can sprout and result in the spread of the plants as well. Dandelions are well-known simple perennials.
Creeping perennials are problems because they live for many years and spread by rapidly growing roots as well as stems. The propagative stems grow horizontally, above or below the soil surface. At each node on the stem, a new weed sprouts upwards. Quackgrass or inula are creeping perennials.
Perennial weeds, especially creeping perennials, are the most difficult to control with herbicides. This is because you are limited to post-emergent herbicides only. The extensive roots or tubers on many perennials keeps them coming back after repeated herbicide treatments.
Weed reproduction
Again, like with all pests, knowing about their reproduction is basic to understanding how to manage weeds. There is really nothing complicated about weed reproduction. The problem is that they reproduce aggressively, in a wide variety of environments.
Much has been written about the reproduction of weeds by means of seeds. It has been said that most weeds produce tremendous numbers of seeds. Is this really the case? How different are weeds in this regard? Consider this. One mullen plant can produce 223,200 seeds. (I wonder who counted them?) Anything over 10,000 per plant is a problem.
Most annual ornamentals have been bred not to produce seed. Their wild cousins produced lots of seed, just like weeds.
Woody ornamentals often produce thousands of seeds.
There is lots of research on the residual survivability of weed seed. Many weed seeds remain viable in the environment for decades. Even Canada thistle’s seeds can last for 20 years, and that is a mild example. Seeds of desirable plants last a long time as well. The problem is that surviving seeds can germinate when they are eventually “put” into a conducive environment by cultivation, irrigation or by growing media preparation.
Dissemination of weed seed can be problematic. There are many that have means of long-distance spread. Again, this is no different from plant seeds in general. But, it is important to know how the most troublesome weeds in your nursery can spread.
Weed seeds are produced at different times of the year. Again, you need to know this for proper weed management. Many herbicides work by preventing seed germination or destroying young seedlings before they emerge from the soil. You need to know what time of the year this is.
Weed seeds vary in their susceptibility to chemical management. You need to scout what weeds you have and make product choices properly. Many weed seeds can germinate and seedlings can develop in very adverse environments.
Many weed seeds have developed aggressive means of long-distance spread. Various weeds can be spread afar by passing through birds or animals that eat the fruit, by wind currents, by water or by sticking to animal’s hair or fur or workers’ clothing.
Many of the most troublesome weeds reproduce by means other than seeds. This limits our choices of management practices. Most of these weeds are perennials. Here are some other types of weed reproduction:
* Bulbous weeds produce bulblets, which are broken off the parent bulb.
* Pieces of the roots, runners or stems of many weeds can be severed and spread during cultivation.
* The creeping structures of some perennial weeds grow very fast, forming a new plantlet at each node on the runner.
Parts of a weed management program
Weed management is not unlike pest and disease management. These programs have four parts:
1. Diagnosis. This often involves scouting and recordkeeping.
2. Planning management strategies. These need to be integrated, using many different tools and approaches.
3. Implementing management strategies. Implementation includes short-range and long-range practices. It includes preventive and rescue practices.
4. Evaluating management strategies. Since management programs like these are dynamic, the outcome of present practices must be known before intelligent decisions can be made on the need to change parts of the program in the future. Good recordkeeping is essential.
Diagnosis and scouting for weeds
The purpose of weed scouting is to determine which weeds threaten your plants. You must determine weed problems that will need more intensive management programs to maintain the quality of your products.
Here are the main points of a weed-scouting program. Recording your findings is extremely important, as that will weigh heavily on the next step in the control program. You should:
* Make a list of the most prevalent weeds you have.
* Determine what life cycle your most common weeds fall into.
* Note if you have any weeds on state or federal noxious weeds lists.
* Note where the weeds are in your nursery. Be especially aware of weeds in non-cropped areas of the nursery.
* Note whether the occurrence of weeds are a result of past cultural practices or weather conditions.
* Note any shifts in prevalent weeds from one year to the next. Note the presence of “new” weeds in your nursery.
* Note which weeds escaped your present herbicide program. The biggest problem with herbicides is that they often permit one or two weed species to survive. This leaves behind a monoculture weed situation. Such monoculture situations can result in rapid spread of the surviving weed.
This information is taken from my new book, “Producing Healthy Plants: A Systems Approach.” A prepublication copy can be ordered on CD for $30 by contacting Chuck Powell,
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- Charles C. Powell
March 2008