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Don't Forget About the Discards: Managing Onion Cull Piles

There is a familiar saying "one person's waste is another person's treasure". Well, the same could be applied to the creatures attracted to those onion cull piles left at the side of the field. An onion cull pile, by definition, is a pile of onions comprised of graded out onions, partially rotted onions and onion waste.

Cull piles of any crop can act as a great reservoir for diseases and insect pests. For example, in potatoes, cull management is critical as a mitigation measure against late blight. In onions, cull pile management is important in the control of onion maggot populations. In order for the season's third generation maggots to survive and overwinter in the field, they must have a supply of onions on which to feed. Cull piles can act as an ideal source for this supply.

In addition to managing pests, now that Iris Yellow Spot Virus has been detected in Ontario in both 2007 and 2008, cull pile management will be a critical step in minimizing the green bridge between growing seasons.

Ways to minimize large onion cull piles include fall and winter spreading, and/or the burial of crop debris. It's important to remember that onions are a biennial crop. Discarded bulbs can and will sprout and grow. One way to minimize the growth of volunteer onions in your cull piles is to chop up the onions. Unlike potatoes, onion pieces do not have the ability to regrow.

Field spreading is an option for dealing with cull onions in the fall and winter rather than a large pile at the edge of the field. The key to spreading is depth. Cull onions spread too deep will not allow for adequate freezing. So, piles should be no more than a foot deep.
It's important to think about where you're spreading your culls, if you decide to carry out this control option. Try not to spread culls on land that you plan on growing onions, or other alliums, in the next year or two.

As mentioned earlier, a second management option with cull piles is field burying. Again, minimize the depth of the pile to prevent inadequate freezing, ensuring at least a foot or two of soil covers over the onion debris.

At the end of the day, handling your onion waste in the right way now will help with insect and disease management next season.

An example of onion cull piles

An example of onion cull piles