Covers subjects about horticulture and pest management for the grower and associated industries What are Weeds? What are my garden plants to keep? So many people ask what are weeds because it’s hard to tell when you’re a beginner gardener Ways to conquer those unwanted or unknown plants from invading your gardens or yards
Weeds And Seeds
The last post demonstrated the remarkable ability and unique features of aphids that allow them to rapidly boost their numbers and colonize their hosts in favorable conditions. What about weeds? What features give them the ability to rapidly colonize a potted crop or planted field? Many plants become weeds because they have the powerful trick of producing many many seeds. Seeds are dispersed by wind, insects, animals, or by their association with our nursery tools and machinery. Often, these seeds are long-lived in the soil. Consider these statistics:
Probably some unfortunate graduate students or field assistants in 1954 were given the task to count weed seed of hundreds of common weed species from about 50 plant families in North Dakota. The table above is just a sampling (Stevens 1957). In most cases a single plant, judged to be of average size and growing where competition was low, was harvested at maturity or when a maximum number of seeds could be obtained. The plants were air dried for two weeks or more, threshed and cleaned to re-move immature seeds, empty florets, etc. All of the sampling methods are described in the Stevens reference given below.
So basically left on their own, weeds have a profound ability to produce seed. Some seed are not viable, some germinate immediately, and some persist, perhaps for years, in the soil as a “seed bank”. This bank represents the holdings of weed seeds in the soil. Place a “deposit” of seed in this bank, and your “interest” is compounded in a big way. An interesting experiment with velvetleaf (Abutilon theophrasti) an important weed in soybean crops demonstrated this (Hartzler, 1996).
Velvetleaf is a prolific seed producer and seeds are long-lived. In 1990, replicated experimental plots were planted with soybean and then with one of three velvetleaf densities: 0, 0.2, and 0.4 plants per square meter. In subsequent years, the experiments were maintained in a corn-soybean rotation. Weed densities were determined at crop harvest for four years. As seen above– even with competition from the crop plants– velvetleaf density increased dramatically for years following the very sparse initial planting of the weed. There were even some velvetleaf plants seen in the untreated “0” plots, even though the plots were hand weeded to reduce seed production for 5 years prior to initiating the study.
The number of weed seeds in in the soil can range from near 0 to over 1,000,000 per square yard, and most weed seeds are between 0 and 5 years old. A small number of seed can remain viable for decades or more. With this knowledge, one of the most important principles of weed management is to “never let weeds go to seed”. Never.
Stevens O.A., 1957. Weeds, Vol. 5, No. 1 (Jan.), pp. 46-55
Hartzler R.G. 1996. Velvetleaf (Abutilon theophrasti) Population Dynamics following a Single Year’s Seed Rain
What are Weeds?
Help!? I have no idea which plants I can eat and which are weeds!
This is a common beginner question and one that I get asked all the time. How do you know which seedlings are your food and which ones are weeds?
Now I know many of you will say: ‘But you can eat many weeds! There are plenty that are edible & healthy for you to consume!’
And you’re right!
However for those that do not wish to consume those edible weeds we’re just going to discuss how you can figure out which of your little seedlings are weeds and which are your crops.
Got weeds? Remove them before they set seed.
Common mullein in its second year of growth. This seed head will disperse around 200,000 seeds. Photo by Rebecca Krans, MSU Extension.
Many gardeners are calling the Michigan State University Extension Lawn and Garden Hotline and uploading photos to our Ask an Expert resource wanting to know if what they’re trying to identify is a weed. A weed is a subjective human classification usually indicating a plant out of place, but identifying a plant you see as a problem is a great first step in finding the right solution for your yard or garden.
For help in identifying weeds, check out the MSU Weed Diagnostic resource for proper weed identification and management tactics, contact the Lawn and Garden Hotline at 888-678-3464 or upload your photos at Ask an Expert. Once you have properly identified what plant it is, then you can more efficiently decide on the best plan of attack. Read on to discover ways to outsmart these unwanted plants.
When do weeds flower?
It is always encouraging to hear a gardener’s “ah ha” moment when realizing weeds have specific life cycles, i.e., they mature or set seed at different times throughout the year. Some are summer annuals, winter annuals, biennials or perennials—review the “Spring blooming lawn and garden weeds” article from MSU Extension to understand this better. Determining a weed’s life cycle will help you manage them better and possibly prevent future occurrences. For example, if you can eliminate the weed prior to seed production or before seed dispersal, then you have made a great effort toward elimination.
Throughout the growing season, take notice of unwanted plants in your garden or yard and remove them immediately. After all, an amazing adaptation of weeds is that they produce many seeds. For example, one common mullein plant can produce at least 200,000 seeds, and one purslane plant can produce two million seeds! No wonder it may seem like you can never get rid of them. Many seeds can live for years within the soil in what is called the seed bank, so it is not only the current year but also past year’s practice that plays a role in how many weed seeds are present. For more reading, MSU research explains “Weed Seedbank Dynamics.”
Weeds have multiple survival tactics
Once you have properly identified the weed, search out its different survival tactics. For example, not only will weeds produce many seeds, but they will also have different ways in which the seed may be carried or transported away from the original mother plant, resulting in less competition among seedlings, thus better survival rates.
Reproduction may also occur vegetatively for some, which means if you leave a portion of a root or rhizome or stolon (i.e., below and aboveground creeping stems, respectively) in contact with the ground, this part will continue to live and regrow. Dandelion, Canada thistle and creeping bentgrass, respectively, are examples with these survival tactics.
Do not dispose these vegetative parts in your compost pile, as they can resprout and be reintroduced back into your garden. Also, try to avoid placing any weed seeds back into your compost. Unless you are actively managing your pile at temperatures of greater than 140 degrees, they may survive and be reintroduced back into your garden.
Weeds have useful properties, too
Weeds can be frustrating, but by better understanding their specific life cycles and adaptations, you are better armed to defend your garden and landscape against them. Be mindful that many of what we term “weeds” were actually brought here because they had useful properties that served human civilization over time, such as food sources, nutrients and medicinal properties.