Michelle Grabowski, UMN Extension
H. Schwartz, Colorado State University, Bugwood.org
Photo 1: Onion seedlings suffering from root rot
Although spring came early in Minnesota this year, recent cold wet weather throughout the state is reminding many gardeners that summer is not here just yet. Many perennial plants, trees and shrubs have been leafed out for weeks now. However, gardeners should remember that when starting warm season vegetables from seed in the garden, it is important to wait for soils to warm up. Pumpkin, cucumber and squash seeds like soils that are 65F at a 2 inch depth. Melons prefer soil temps of 70F or above. Sweet corn seeds germinate best when soils are between 55 and 60F. Soil temperatures are measured weekly by the UMN Climatology group at several sites across Minnesota and can be viewed online
Seeds planted into cold soils may sit and wait or may germinate but grow very slowly. In these very early stages of life, seedlings are highly susceptible to soil borne pathogens that cause root rot and damping off. Gardeners may notice that seeds are failing to emerge from the ground or that young seedlings emerge and then yellow and fall over. Older plants may become stunted by root rot. Leaves may wilt and dieback. The recent excess of heavy rain in many areas of the state has created ideal conditions (cool, wet soils) for these root rotting pathogens. Several cases of seedling damping off and root rot of young plants have been reported.
Dept. of Plant Pathology Archives, North Carolina State University, Bugwood.org
Photo 2: Impatiens transplants suffering from root rot
To prevent these early season root rotting pathogens, wait to plant warm season vegetables until soils have warmed to the recommended temperature and are moist but not heavily wet. In some areas of Minnesota, it may be necessary to start seeds indoors and transplant them into the garden when outdoor conditions improve. In addition many simple structures like cold frames and walls of water
can help warm soils and protect plants. Black plastic mulch also warms soil in the root zone. By growing plants in a raised bed, gardens have improved drainage even in wet weather.
Seeds coated with a fungicide treatment will be protected at the earliest stages of growth. For gardeners that choose not to use treated seed, waiting until soil warms up to the appropriate temperature or using the techniques listed above to heat soil and improve drainage can be just as effective in preventing early season root rot.