Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
We have had large losses of cantaloupe and watermelon transplants this spring due to the cold, rainy, and cloudy conditions and several WCU articles have been written on the subject (see Dr. Kate Everts article this week).
To more fully explain this problem, it is necessary to understand how different vegetables regenerate roots and how this affects plant survival after transplanting. As has been discussed previously, soil temperature is very important. Rate of root growth or regeneration is temperature dependent with cool season vegetables such as cabbage or lettuce being able to produce new roots at much lower temperatures than warm season vegetables such as eggplant or watermelon. In soils that are below critical temperatures (60-65°F for watermelon and cantaloupes for example) roots do not grow into the soil bed and transplants will be subject to desiccation losses as soils dry around the root ball. The smaller the root ball (the smaller the tray cell size), the more quickly desiccation and plant loss can occur. For Solanaceous crops tolerance to cold soil is as follows Tomatoes > Peppers > Eggplant. For cucurbits tolerance to cold soils is in this order Cucumber > Summer Squash > Muskmelon = Watermelon.
A second problem relates to where plants can grow or regenerate new roots from. Solanaceous vegetables (tomatoes, peppers, eggplant) can generate new roots from both the existing transplant root system and also from stem tissue. Stem generated roots are called adventitious roots and in solanaceous transplants they can grow at any place along the stem above the root system. There is still some bare root transplant production of solanaceous crops because of this ability to regenerate roots.
Adventitious roots on tomato transplant stem.
Adventitious roots on stem of tomato just starting to form
In contrast, cucurbit transplants will only generate adventitious roots at above-ground nodes and no nodal tissue will be in contact with soils at planting time in the spring. Therefore, all new roots in cucurbits must be generated from the existing root system. Cucurbit root systems that are damaged (torn or detached) during transplanting will not survive (Solanaceous crops will). Cucurbit crops must be firmly rooted in the plant trays so they will pull out with no tearing, otherwise plant losses will occur.
Cucurbit transplants will only grow from existing roots in the root ball (circle). Adventitious roots are only generated at nodes (arrow) and will not form on new transplants.