Jun 20, 2019
Consequences of a cool, wet season for vegetables

The consequences of a cool, wet start to the growing season go beyond the obvious of later planting and slow growth.

This is the most difficult season in recent memory. Temperatures and sunlight have been below normal while rainfall has been above normal. The consequences of a cool, wet start to the growing season go beyond the obvious of later planting and slow growth.

Irrigation, fertilization and plant growth

These three are closely connected. With continued and excessive rain, irrigation is a non-issue even in black plastic systems. However, in a black plastic system, growers rely on putting fertilizer through the drip system. This should continue, but run the system no longer than it takes to inject and disperse nutrients. Minimizing irrigation maintains nutrients within the root zone and keeps them available for when normal conditions return.

This poses difficulty for pepper growers who apply most of their nitrogen prior to first fruit set. It is important on peppers to get a substantial plant prior to fruit set since the plants are not capable of developing fruit without affecting plant development and future yield. Therefore, in a normal year, many pepper growers will apply 75% of their total season nitrogen prior to first fruit set.

Sweet corn growers also need to apply additional nitrogen since much of the pre-plant nitrogen has leached away. Many corn fields are showing this already with a patchwork of yellow-green and green, with plants having a deeper green color in areas better able to retain nutrients. Go to “Water management in a plasticulture system: What we learned from the okra incident” for more information on irrigation in a plasticulture system.

Weed control

Many chemical preemergent weed control products do not work as well in cool, wet conditions. A couple factors come into play. First, the products work by maintaining the appropriate concentration within a thin layer at the surface. If the product moves deeper into the soil with excessive rain, it is diluted to the point that control is affected and, driven deep enough, may even be detrimental to the commercial crop.

The other factor is that when it is cool, many summer weeds do not germinate until after the concentration needed for good control is low enough that the product does not perform well. Therefore, expect more weeds to be a problem as the season progresses.


The main ingredient for successful season-long disease control is to minimize early infection. The next is to get plants in the field as soon as possible. Spreading plants out minimizes plant-to-plant contact, reducing disease spread. Many transplants this year went from the greenhouse to holding areas for several days before fields were ready for planting. This kept thousands of plants in extremely close contact under wet conditions. This is ideal for disease spread, especially bacterial diseases of pepper and tomato. Use the highest rates recommended for the disease control products, at least until you are able to assess if you have issues or until the weather turns drier.

The conditions we have experienced are also ideal for early spread of downy mildew. Vine crop producers should refer often to MSU’s Downy Mildew News so you can observe if downy mildew is a threat in your area.


Honeybees do not like to work in cool, wet weather. Early flowering, vine crop plantings may have an increased number of poorly pollinated fruit, even if hives have been brought into the field. Good pollination and fruit development requires several bee visits and this may not happen with temperatures below or near 65 F or in cloudy, wet weather.

Plant growth

Plant growth will be slower under the conditions we have experienced. Plants are subject to ambient temperatures and their physiological processes are slower when the temperature is low. They are not able to take up nutrients as readily under cool conditions, further slowing growth. Expect later than normal harvests, however, for tomato fruit may be larger. Hot temperatures often cause tomato fruit to ripen before they reach full size. Cool temperatures allow the fruit to get larger before it begins to ripen.

The bottom line is that even if the weather improves, be aware that there will be consequences through the entire season and to not let your guard down. In fact, you will need to be more diligent than ever.

MSU Extension offers additional educational resources and programs to help farmers as they deal with delayed planting seasons at https://www.canr.msu.edu/agriculture/delayed-planting-resources.

Photo: Kurt Stepnitz, MSU Photography

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