8 years ago I chose the location of my garden based on two factors. The first being that a house was demolished years ago on one end of the property, so I wanted to put it far away from any hidden debris and avoid the clay that was turned up during the demolition. The second, was that my hose had to reach. In the end I really didn't have much choice where it went. I had no idea that I had to take into consideration the trees growing near my garden. I found out, however, after a crappy first year of tomatoes and a conversation with the lovely gal who runs the garden center, that the young black walnut tree right next to my garden was likely the culprit.
Black Walnut branch with nuts.
Its called alleopathy. And it's an interesting trait where some plants produce one or more biochemicals that affect growth, germination, and reproduction of other plants. Known as alleochemicals, they can either support the growth of neighboring plants or kill all the competition. Additionally some plants use alleochemicals to protect themselves from being eaten by animals. Black walnut trees produce a biochemical called “juglone” that kills many types of plants (including tomatoes) in an effort to ensure that the tree can get the most nutrients from the surrounding soil with little competition. The biochemical works by inhibiting respiration, and essentially starving out sensitive plants. Juglone exists in all parts of the tree, bark, roots, leaves, and of course the walnut hulls. Underneath the tree's canopy is where most of the danger lies, however it can be as far-reaching as 50-80 feet from the trunk of the tree. Even if the tree is cut down, it will be years before the toxicity declines, as it is impossible to remove all of the roots from the soil.
A look up into the canopy of a young Black Walnut tree.
Plants suffering from juglone toxicity will show wilting and yellowing of the leaves that doesn't improve with thorough watering. And eventual death of the plant. There are many lists of juglone-sensitive and -tolerant plants out there and this is a very short one at that. There is also some debate about what plants belong on the list and ones that don't. It seems that some varieties of tulips, for example, are juglone tolerant, while others still are sensitive and won't survive near the trees. Most of the studies regarding juglone sensitivity have focused on tomatoes and a few other popular crops. Check out state extension services for more lists.
Juglone Sensitive Plants:
Juglone Tolerant Plants:
I admit that my juglone sensitive plants have still done quite well despite this toxin being present in the soil, besides peppers, that is, never did have much luck with peppers. I attribute my success to not allowing leaves/twigs nor nuts and hulls to decompose in my garden. Also, my whole garden gets tilled twice a year, and in between rows several times in the season.. The aeration and well drained soil goes a long way in aiding the decomposition of juglone. Making sure your garden is nutrient-rich and has lots of healthy microbes from lots of compost helps a lot, too, but it only delays the inevitable. As the tree grows the toxin will spread and I will be forced to take action. Luckily there are ways to get up and away from juglone. Raised beds lined with weed/root proof liner or even container gardening are possibilities, along with the obvious: Moving the garden.
Since juglone exists in all parts of the tree, be sure to never ever compost any part of the tree and never use it as mulch. Black Walnut trees aren't the only juglone-producing tree you need to look out for either. Butternut, hickory, and English walnut are also guilty of producing the toxin. So if your garden is a little sad and wilted,, look up and take a gander at the trees that surround it.