Mark your calendar. June 13th is National Weed Your Garden Day. It’s time to get out into your garden to remove those unwanted plants growing in amongst your prized herbaceous ornamentals and tasty vegetables. Not only will weeding make your garden look more tidy and beautiful, it will also help make your favorite garden plants healthier. How do weeds negatively impact your garden plants’ health? Let’s look at some ways.
Weeds compete for nutrients and water: All plants in a garden use and compete for available soil nutrients and water to grow, flower, and set fruit and seed. Nutrients and water used by weed plants are not available to be used by ornamentals and vegetables that we are attempting to grow. This leads to smaller plants, fewer flowers, and reduced vegetable yields. In addition, plants stressed for nutrients and water because of competition from weeds are less likely to be able to produce compounds that they can use to fend off infections by the myriad of plant pathogens in the environment. This can contribute to an increase in infections and disease, leading to a further reduction in plant quality and yield.
Weeds create an environment that is more favorable for infections to occur: Weeds, like all garden plants, produce foliage. Thus, high weed pressure in a garden will lead to a denser plant canopy that will reduce airflow. When leaves get wet (e.g., when it rains), this lack of air movement will increase the length of time that it will take for the leaves to dry. Longer periods of leaf wetness will provide more time for spores of fungal plant pathogens to germinate and infect, thus increasing the likelihood that many types of leaf diseases will develop.
In addition, plants (including weeds) transpire. Transpiration is a natural loss of water from leaves as they take in carbon dioxide and release oxygen during photosynthesis. Dense plant canopies trap this released moisture, creating humid conditions around leaves. High humidity tends to promote sporulation of many disease-causing fungi, which can lead to an increase in additional infections. For some disease-causing fungi (e.g., powdery mildew fungi), high humidity (rather than leaf wetness) is the environmental factor that promotes spore germination and infection.
Weeds can serve as a reservoir for disease-causing organisms: Many disease-causing organisms are very host specific, in that they can infect only a single type of plant or only a small group of very closely related plants. Other pathogens have broad host ranges and can infect many types of plants. Weeds (particularly perennial weeds) can serve as reservoirs where these broad-host-range pathogens can overwinter and subsequently spread to garden plants. In particular, I worry about weeds harboring viral pathogens such as Tobacco mosaic virus (TMV), Cucumber mosaic virus (CMV), Impatiens necrotic spot virus (INSV), and Tomato spotted wilt virus (TSWV). These viruses can survive in weed hosts and can subsequently be moved be moved from plant to plant by insects, on gardening tools, or even by just handling plants. Weed species can also serve are reservoirs for the phytoplasma that causes aster yellows, and they can help keep fungal pathogens such as Verticillium (the cause of Verticillium wilt) at elevated levels in garden soils. By weeding, you can eliminate sources many disease-causing organisms and help prevent pathogen survival and spread.
So, as you get some spare time in the coming weeks, be sure to spend a few minutes in your garden removing those pesky, unwanted plant species. You will not only end up with a more beautiful garden, but ultimately a healthier garden as well.
If you have questions about weeds and how they can impact diseases in your yard, feel free to give me a shout. As always, you can reach me at firstname.lastname@example.org or (608) 262-2863. Go forth and weed!!