I saw on a recent publication the suggestion to plant an onion with potatoes to ward off a specific beetle. Two questions: 1. Is this appropriate when growing potatoes in a container or just in the ground?; 2. Can I use a grocery store onion (I know potatoes are suppose to be seed potatoes but not sure about onions)
Thanks for contacting the Toronto Master Gardeners.
I have not been able to find any research that suggests that onions planted with potatoes will ward off insect pests. The plants that are recommended for companion planting with potatoes are the following, most of which would not be suitable for planting in the same container because of their mature size: beans, broccoli, Brussels sprout, cabbage, cauliflower, Chinese cabbage, corn, eggplant, horseradish, kale, kohlrabi and pea. Green beans help to deter Colorado potato beetles, so you might consider planting them close by. (Frankie Flowers, “Food to Grow”, Toronto 2016)
If you are growing your potatoes in a container you will be limited in the size and number of companion plants you can introduce. Because potatoes grow deeply in the soil, you might consider shallowly-rooting plants with above ground growth, such as lettuce, spinach, scallions (green onions) or radishes, which will not take up much space, and because they are harvested earlier in the season, won’t be competing with the potato harvest. You could also think about planting herbs such as parsley, basil or thyme.
Incidentally, you are quite right about buying certified seed potatoes for planting. This is to prevent late blight, a devastating fungal disease of potatoes and tomatoes. Here is a little background from a previous Toronto Master Gardeners post.
For a further discussion of companion planting, please see the Toronto Master Gardeners’ Guide: