Low Soil Temperatures and Transplant Establishment

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

I have had several growers and consultants report watermelon transplants that have poor root growth. They wondered if a disease was causing the damage. In several cases there were no pathogens involved and the seedlings seemed to be affected by the low soil temperatures following transplanting to the field. There was an excellent article in the Vegetable Crops Hotline out of Purdue on this topic

https://vegcropshotline.org/article/establishment-failure-of-watermelon-and-cucumber-transplants-because-of-low-soil-temperatures/ and Gordon Johnson also wrote an excellent article in an earlier WCU issue on transplant potential when plants are set into cold soils http://extension.udel.edu/weeklycropupdate/?p=8993.

Tips from the from the VCH and WCU articles for watermelons:

  1. Do not plant watermelons if anticipated soil temperature is below 65°F at the time of planting;
  2. If possible, use transplants that have developed a solid root ball;
  3. Lay plastic early in the season to ensure adequate heat accumulated under plastic;
  4. Make sure there is good plastic to soil contact to aid in heating the soil.

Our soils, especially in early May were unusually cold due to both cold weather and prolonged overcast conditions which failed to warm up soil under plastic. Soil temperatures at the University of Maryland Research and Education Center in Salisbury for the first 10 days in May are listed in the table below. These temperatures are the average daily temperatures in bare ground. Plastic would have increased temperatures, but only under sunny conditions. Temperatures of 65°F were reached only two days at the 2” depth and one day at the 6” depth.

Soil Temperatures at the UMD Research and Education Center, Salisbury, MD in Early May, 2016

Date Soil temperature 2” depth Soil temperature 6” depth
May 1 56.4 57.1
May 2 65.0 61.0
May 3 67.1 65.0
May 4 59.7 61.3
May 5 56.5 58.3
May 6 55.6 56.9
May 7 59.4 58.3
May 8 63.3 61.0
May 9 59.6 59.7
May 10 62.8 61.5
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