Food and the Future Environment

Weed Survival Characteristics


Weed Survival Characteristics

In addition, many weeds have traits that enhance their survival and reproductive success such as: i. hard-seeds or seeds that can remain dormant for long time periods until environmental conditions for germination are good, enhancing weed seed success, ii. plant protective characteristics such as thorns, toxic tissues, protected growing buds, iii. adaptive growth to a wide range of environmental conditions also referred to as plasticity. For example, in a field or lawn that is grazed or mowed to a short height to control weeds, adaptive weeds can produce leaves very close to the soil surface and flowers on short stems below the mowing height.

Figure 8.2.6: Buttercups (Ranunculus L. ) contain protective compounds that are toxic to most grazing livestock. For more information about buttercups see Creeping Buttercup, Pacific Northwest Extension Publications.
Photo: Heather Karsten
Figure 8.2.7: Thistles with protective thorns.
Photo: Heather Karsten

Check Your Understanding

Read about the Velvetleaf weed species (Abutilon theophrasti L.) at Velvetleaf.

Velvetleaf has hard seed. How long can the seed survive? On the website click on the link that discusses Velvetleaf Adaptation and Stress. What examples does the author use to illustrate velvetleaf plasticity or ability to adapt to its growing conditions?

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ANSWER: Velvetleaf can survive up to 50 years. Examples of plastic or adaptive growth include: i. when resources are NOT limited, a velvetleaf plant grew short and produced many seed capsules, ii. when many velvetleaf plants germinated close together, the plants grew tall to compete for light. iii. when a velvetleaf plant was growing along a roadside with poor soil conditions where it was frequently mowed, it still managed to produce a small plant and seeds.

Activate Your Learning: Weed Control Practices

Recall what you have learned about crop plant lifecycle classification and characteristics in Module 6.

Read the Australian Department of the Environment website that describes Integrated Weed Management. Click on and read the links that describe each type of weed management technique. After you have read both of the above readings, answer the questions below.

Question 1 - Essay

Review examples of the four weed control strategies discussed in these two online publications. Explain at least two specific weed control strategies that are likely to be effective for controlling annual weeds and explain why they are effective for annual weeds.

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Annual weeds typically germinate, grow and develop to maturity and seed production quickly, and therefore can offer significant competition with crop plants for light, nutrients, and water. They can also produce seeds and increase weed population pressure relatively quickly. Therefore, annual weeds should be terminated early and especially prior to they produce seeds. Because annual weed species do not allocate significant resources to below ground storage organs, they can be terminated with mechanical or physical control tactics such as: plowing, cultivation, hoeing, removal by hand, hay making, mowing or grazing, soil mulching, and flaming. Allowing weed seeds to germinate and then terminating them with light tillage (stale seedbed) strategically uses tillage for weed control. Annual weed germination and establishment can be suppressed with cultural control practices such as crop rotation, rotating crops with different seasonal life cycles, successive planting (double or triple-cropping); the integration of cover crops; and managing for competitive crops with early crop planting and good crop management practices (competitive crop varieties, soil fertility and health management).

Chemical control with herbicides applied at the recommended time, and rotating or varying herbicide chemistry can reduce the evolution of herbicide-resistant weeds. Biological control can also reduce weed populations and may include conserving habitat for weed seed predators such beetles, small rodents, and birds by integrating cover and perennial crops on a farm, and avoiding pesticides that can reduce weed seed predatory populations. In addition, in some cases, browsing or grazing animals or specific pathogens of weeds such as bacteria or fungi are sometimes introduced.

Question 2 - Essay

Describe at least two weed control strategies that are likely to be effective to control perennial weeds. Explain why.

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Perennial weeds have below-ground storage organs that they can regrow from (ex. tubers, rhizomes, stolons, and bulbs); therefore mechanical control strategies such as tillage, cultivation, and hoeing can break up and distribute perennial weed storage organs, facilitating the spread of perennial weeds. Mechanical weed control tactics can be effective if they can bring the majority of the storage organ to the soil surface to desiccate or freeze, thaw, desiccate and die. Perennials typically begin storing reserves for spring regrowth in late summer and early autumn. Therefore, frequent mowing or flaming can deplete a perennial weed’s storage reserves, if the tactic is repeated multiple times, particularly during summer before the plant begins replenishing storage reserves.

Applying systemic herbicides that are taken up by the plant and translocated to storage organs is also most effective in late summer and early fall when perennial plants are replenishing storage reserves. Rotating or varying herbicide chemistry can also reduce the evolution of herbicide-resistant weeds.

Cultural control strategies for perennial weed control include crop rotation between perennials and annual crops, planting perennial crops with competitive annual companion crops that are harvested early, and managing for competitive crops with early crop planting and good crop management practices.

Biological control practices as described for annual weeds can reduce perennial weed populations, and be particularly cost-effective in rangelands where other practices are often cost-prohibitive. Biological control strategies may include introducing browsing animals such as goats that will eat thorny perennial weeds, or application of weed-specific pathogens such as bacteria or fungal spores.