Master Gardeners: Several plants tolerate the toxic black walnut

What can be grown under black walnut trees? As you have probably heard at some point, black walnuts produce juglone toxin which is deadly to tomatoes. So we are cautioned not to plant those crops under or near black walnut trees. Not only are the...


What can be grown under black walnut trees? As you have probably heard at some point, black walnuts produce juglone toxin which is deadly to tomatoes. So we are cautioned not to plant those crops under or near black walnut trees.

Not only are the roots of the black walnut toxic, but when listening to WCCO at 8 a.m. on Saturday morning, we learned that rain dripping from the leaves of the black walnut will contain the same juglone. So this means toxin falls out of the drip line of the tree. Keep this in mind if your garden is located anywhere near black walnut trees.

We have had gardens near our black walnuts for nearly 40 years.

LaVonne kept her cold frames under her black walnut tree for over 30 years with no bad results. Granted, the plants were in pots and not direct-seeded, but they would have received rain drops coming off the tree.

Sue's vegetable garden has been planted next to three rows of black walnuts and she has noticed no problem. (It most certainly doesn't slow down weed growth either!) Her raspberry patch ended up being partially shaded by these trees with no ill effects. And she has a Haralson apple tree that is shaded to the north by another black walnut. The shade probably affects fruit production more than any toxin coming from the tree.


Over the years Extension master gardeners have been compiling a list of what plants do not seem affected by the toxic juglone. We would like to share this list.

Vegetables not affected include squash, melons, beans, carrots, corn and black raspberries.

Flowers not affected include hollyhock, Jack-in-the-pulpit, astilbe, wax begonia, calendula, campanula, crocus, dutchman's breeches, snowdrop, sweet woodruff, cranesbill, coral bells, morning glory, bee balm, garden phlox, Kentucky bluegrass, lungwort, bloodroot, lamb's ear, spiderwort, zinnia, pansies, violets, daylilies, hosta and sedum.

While we are on the subject of trees, we recently came across some interesting information.

If you need to have a tree removed from your yard, and even if you have the stump ground out, any remaining roots of the removed tree will biodegrade and in that process will pull nitrogen out of the soil.

For this reason it is not advisable to plant a replacement tree in this same area because the remaining roots will be a problem for a number of years until the old roots have completely rotted. If you have already done something like this, be certain to add a lot of organic nitrogen fertilizer to the area.

Poison ivy

When you are out communing with nature, keep a watch out for poison ivy. It is a perennial plant that often is low growing between 1 and 2 feet tall but can also be a climbing form and can be 3 to 12 feet or more in length.


"Leaves of three, let it be" is a place to start when it comes to identification. Leaves tend to be 2 to 7 inches long and 1 to 4 inches wide with an egg shape. They have a pointed tip and smooth to slightly irregular toothed edge. They have a strong mid-vein running up the center with smaller veins evenly spaced at a 45-degree angle.

These leaves are glossy-green with a possible red-purple hint in early season. In the fall the leaves turn yellow to red. Some plants will produce a cluster of white to cream-colored berries. The plant spreads by its roots, above ground vines or berry distribution.

Beth Berlin, Extension Educator with the University of Minnesota, tells us to control poison ivy, look for herbicides containing triclopyr. It is often under labels specifically for poison ivy or a woody brush killer. Apply when temperatures are 60 to 85 degrees.

Read and follow all label directions. Avoid spraying on windy days to prevent drift. Reapplication may be necessary a few weeks after initial treatment as it is a tough plant to kill and may resprout. Do not treat in the fall but when actively growing.

Master Gardeners Sue Morris and LaVonne Swart started writing this column 26 years ago, in 1991, for Kandiyohi County newspapers. They are both certified through the University of Minnesota as gardening and horticulture experts, Swart since 1981 and Morris since 1983. Both live in Kandiyohi County. To consult with a Master Gardener, call your county Extension office.

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