Q: We are building a raised bed in the backyard for vegetables. Should we line the bottom with cardboard? What should we fill it with? Also, our neighbors have a black walnut tree. Will that harm our plants?
A: We’ve been getting lots of questions from people building raised beds this spring. Many people want to start vegetable gardens, and raised beds are a great way to go. It’s easier to keep them weed-free, you don’t have to till them, they help deter pests, and the soil warms up faster in the spring. In my own yard, which has heavy clay soil, it would be really tough to grow vegetables if I didn’t have raised beds.
There is usually no need to line the bottom of a raised bed with anything. You may see some websites recommending cardboard or newspaper on the bottom, but research shows that a cardboard layer can interfere with soil health. If you have had problems with mice or voles digging their way into your garden in the past, you may want to line the bottom of the bed with hardware cloth or chicken wire.
Fill the beds with a mixture of soil and compost — about 70% soil and 30% compost.
As for the black walnut tree, yes, that may be a problem. Black walnuts produce a substance called juglone that can harm or kill susceptible neighboring plants. Unfortunately, tomatoes and other plants in their family — peppers, eggplants and potatoes — are susceptible to juglone. Cabbages, asparagus and rhubarb are also sensitive to the chemical. The juglone is released into the soil from the roots of the black walnut, and it’s also contained in leaves and nut husks that may fall on your garden beds.
The best thing to do is make sure your bed is at least 50 feet away from the black walnut tree. Eighty feet would be safer. If that’s not possible, you may want to build a barrier in the bottom of the bed to prevent the black walnut roots from growing up into your bed. You can also consider growing vegetables that are less sensitive to juglone, such as beans, beets, carrots, onions, garlic, corn, cauliflower and squash.
There’s more about growing vegetables near black walnuts here: canr.msu.edu/news/growing_vegetable_gardens_near_black_walnut_trees.
Written by U of M Extension Master Gardeners in St. Louis County. Send questions to firstname.lastname@example.org.