Dec 11, 2015
Organic carrots require diligent weed management

Carrots are one of the most popular vegetables sold organically, with 14 percent of U.S. carrot production under certified organic production. However, this crop is also one of the most difficult to produce organically.

“Slow to germinate and establish, this crop requires proactive and diligent weed management – a difficult task for organic farmers to achieve,” Erin Silva, University of Wisconsin organic production specialist and assistant professor in plant pathology, said during a presentation at the 2015 Wisconsin Fruit and Vegetable Conference.

A key aspect of weed management in any organic crop is good stand establishment and canopy cover to shade and smother weeds, preventing them from germinating and establishing, Silva said.

“With carrots, this starts good seedbed preparation,” Silva said. “Even before the ground is prepared prior to carrot planting, cover crops can add organic matter and improve soil tilth, facilitating the formation of a good seedbed for planting a small-seed crop such as carrot.

“In northern climates, winter-killed cover crops such as oat can be particularly useful in achieving these goals. Planting winter-killed cover crops in late summer on ground that will be used for carrots the following year allows for a farmer to get onto that ground earlier than when using overwintering cover crop options (such as cereal rye), while still deriving the benefits that cover crops provide.”

Early season weed management can be tricky for organic farmers producing carrots, Silva said.

The first tool that organic farmers can use comes before the crop is even planting – and this is the tool of stale seed bedding. With this technique, prior to preparing the seedbed, weeds are allowed to germinate and then killed with minimal soil disturbance.

“For example, in the instance of using an oat cover crop, the cover crop may be worked one or two times, and then the weeds allowed to germinate,” Silva said. “After conditions allow for the first flush of weeds to germinate – i.e., after an early rain or irrigation event – the weeds are killed as they are just starting to emerge, typically with a flame weeder or a basket cultivator.  Soon thereafter, the carrot crop is planted into the weed-free bed with as little soil disturbance as possible.

“If basket-weeders or other mechanical cultivation tools are used with this method, it is important to set the equipment to work as shallowly as possible, minimizing the possibility of bringing new weed seeds to the soil surface,” Silva said. “Often, two stale-seedbed cycles, five to 10 days apart, can be effective, with the second pass used to level the beds just prior to planting. Timing of these cycles, though, depends on types of weeds present in the field, as well as soil moisture and temperature – try to time the second pass just when the second flush of weeds is emerging and close to carrot planting.”

Flame weeding works

Flame weeding can also be effective for pre-emergent weed control in organic carrots. This tool is especially effective in slow-germinating crops such as beets, carrots and onions, Silva said.

“Flame weeding occurs just before the seedlings emerge, but when fast-germinating weeds have already begun to establish,” she said. “Like in stale seed bedding, timing is important – coming in too late with flame weeding can injure the crop and result in poor stands. One trick to anticipate the right time for flame weeding involves placing a sheet of plexiglass on the crop row.

“By warming the soil more quickly than the surrounding, bare-carrot row, the seeds under the plexiglass (sealed around the edges with soil) germinate more quickly,” Silva said. “When carrot seedlings are observed under the glass, flame weeding of the row should occur. It is better to err on the side of earlier versus later with flame weeding, however, to minimize the potential of damaging the crop.”

Basket weeders or torsion/spyder weeder combinations are useful pieces of equipment to use for managing weeds after the carrot crop has established. Basket weeders, as they tend to shallowly disturb the soil, are particularly useful when the carrot seedlings are small and the weeds are two leaves or less.

With this tool, rotating baskets spin across the row, flipping up the first inch of soil on either side of the carrot row, without throwing the soil into the vegetable row. The baskets are belly-mounted just under and in front of the tractor operator’s seat, so that the driver can avoid disturbing the delicate seedlings.

A spyder/torsion weeder combination, such as those available through Bezzerides, has springing action that moves soil away from the plant, which is appropriate for delicate crops or crops such as lettuce, where minimal tossing of soil onto the crop is desired, Silva said.

Sufficient water essential

 Another key to obtaining a good carrot stand that will minimize weed growth and maximize yields is adequate watering during crop establishment, Silva said.

“Regular irrigation (sometimes twice per day in hotter parts of the season), in frequent but low volumes, should be done from the time the crop is sown to until the cotyledons have emerged. Especially on heavier soils, this will help prevent crusting, which will inhibit the emergence of the delicate seedlings. As the carrot tops begin to develop more leaves, adequate soil moisture should still be maintained, but with less frequency in volume.”

Regular maintenance of soil moisture will help prevent cracking of the roots, which can occur if watering is irregular or comes in excessively high volumes. Drip irrigation can be valuable to help minimize foliar disease in carrot, such as alternaria or cercospora leaf blights.

“Solid stand establishment and maintenance of good tops benefits an organic carrot grower all season long,” Silva said. “Not only do good, solid, vigorous tops help suppress weeds – one of the most challenging aspects of organic carrot production – but it also ensures a strong stand that can hold up to mechanical harvesting.”

Gary Pullano






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