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field sown with seed and weeds

Winter annual weed seeds that have been dormant during the summer begin to germinate in the fall following vegetable crop harvest. These species overwinter and can quickly form a mat in the spring that can interfere with field preparation, slow soil warm-up, and compete with crop growth. Some winter annual weeds can also be important hosts of pests of vegetable crops. For example, recent studies suggest that shepherd’s-purse and field pennycress (photo 2) may host insects (e.g. swede-midge) and diseases (e.g. black rot) of mustard family vegetables. In reduced tillage systems, winter annuals can be particularly problematic. For example, spring chisel-plowing and glyphosate applications are ineffective at controlling fall-germinating corn chamomile (daisy), which can be a costly contaminant in crops such as peas the following spring (photo 3).

Fall is an important time to evaluate holes in weed management programs and note the location of problematic weeds in order to help crop rotation and weed management planning for next year. This is also a good time to take steps to minimize the number of weed seeds added to the soil, and to prevent establishment of winter annuals.

As crops come out of the field, weeds that are left behind often sow seeds of future problems. For summer annual weeds like pigweed and lambsquarters, the short days of late summer stimulate seed production. Even small plants can produce thousands of seeds at this time of year which can last for decades in the soil. Larger plants can produce from a few thousand seeds (e.g. velvetleaf, smartweed, crabgrass) to 300,000 or more (e.g pigweed, lambsquarters) per plant (photo 1). Seeds of many weed species can last for decades in the soil, creating weed management headaches for years to come. To minimize future weed problems it is therefore important to till or disk fields as soon as possible after harvest to prevent seed maturation. Mowing can also be affective in slowing and reducing seed production, but growth and seed production from lateral branches can be problematic. However, once weed seeds are shed on the soil surface, tillage may be counterproductive, since seed burial protects seeds from predation and decay. Weed seeds are a significant food source for many insects and bird species and can have rates of mortality of 75 percent or greater if left on the soil surface during the fall.

Avoiding summer annual seed production

To reduce problems with winter annual weeds, late fall herbicide applications or tillage are often more effective than spring operations. While it is too late this season, cover crops like oats or mustards established after early harvested vegetables are very effective at preventing winter annual weed establishment. For late-harvested vegetables thickly sown winter cover crops like rye, wheat, or hairy vetch can also help suppress winter annual weeds.

Weed seeds provide a major component of many insect diets this means weed seedbanks can be decreased naturally by encouraging insect (predominantly ant) predation.

Burning can reduce the surface seed banks of many weeds. All crop residues (canola, wheat, lupin and others) can produce a sufficiently heated burn to kill weed seeds. A narrow windrow will burn at a higher temperature and improve weed seed kill.

In suitable soil types, weed seed burial is an effective method of killing weed seeds particularly if herbicide resistant weeds are problematic.

Issues to consider

Inversion ploughing is used to fully invert the soil to ensure weed seeds that were on or just below the soil surface are placed at a depth where they cannot germinate. This can be practiced every 8-10 years, with conservation tillage used in the intervening years. In WA, annual ryegrass seeds failed to establish and eventually died when soil was fully inverted to a depth greater than 20cm using a specialist mouldboard plough fitted with skimmers. This single soil inversion event reduced annual ryegrass numbers by more than 95% at Katanning and Beverley, Western Australia, for a period of two years.

Because weeds that germinate after an autumn tickle can be controlled, such a process will ultimately deplete weed seed reserves. A delay between the tickle and seeding is necessary to give an opportunity for the weeds to germinate and then be killed using a knockdown herbicide. This may cause a yield penalty for some crops.

Please note: This content may be out of date and is currently under review.

This allows greater germination of weeds in particularly weedy paddocks, which can then be killed using a knockdown herbicide or cultivation prior to crop sowing. The longer the delay in sowing, the more weeds germinate and the higher the kill. However, a yield penalty is experienced when sowing is delayed.