Walnut trees in your home landscape are under threat of the invasion by thistle weeds that compete for your plants' nutrients and water. Walnut trees and thistles thrive in similar environments, making walnut trees that much more vulnerable to invasion. Maintain consistent care for vigorous trees and remain observant to avoid a severe problem in your landscape.
Healthy, vigorous walnut trees have a greater ability to avoid and recuperate from thistle invasion and competition than weakened trees. Grow walnut trees in areas that provide full sun exposure. Walnut trees thrive in moist soil rich in nutrients, according to the University of Florida IFAS Extension.
The culture of a walnut tree promotes thistle infestation. Canada thistle (Cirsium arvense), in particular, is a highly invasive weed of trees, according to North Dakota State University Agriculture. These creeping perennial weeds exhibit the most competition in moist, deep, cool soils. Their need for moisture and cool temperatures coincides with cool soils provided under a walnut tree canopy as well as the walnut tree's need for moisture.
Thistles continue spreading season after season as they are perennials and do not die after their first growing season. Through flowering and seed set, thistles continue spreading and forming new root systems that compete with nearby walnut trees. As weeds essentially steal the water and nutrients your walnut tree needs for optimal health, the vigor of your tree declines. A tree in decline becomes more vulnerable to pest and disease problems, so managing thistles is necessary.
Add a layer of mulch to the area surrounding your walnut tree without allowing it to make contact with the trunk. Use an organic mulch such as pine needles, wood chips or compost. Mulch aids in moisture retention and blocks sunlight from reaching weeds, leading to the inhibition of weed formation and suffocating those that have begun developing. Additionally, mulch is beneficial as it keeps soil warm as temperatures drop during cooler seasons, according to North Dakota State University Agriculture.
For thistle growth near walnut trees, employ chemical control when problems become severe. While you can attempt to dig out and remove thistles by hand, it may be difficult to manually manage extreme infestations. Choose an herbicide with the active ingredient clopyralid for effective control of thistles near walnut trees. Apply at a rate of 1 pint per acre during spring after shoots develop or during autumn, according to the Colorado State University Extension.