Gordon Johnson, Extension Vegetable & Fruit Specialist; firstname.lastname@example.org
Watermelon transplants that have an extended hypocotyl, that area of the stem below the cotyledons (seed leaves), can be more susceptible to damage during and after transplanting. Additional stretching of the epicotyl and internodes above the cotyledons can also occur, causing further problems in planting and with plant survival. What is desired is a compact plant with a short hypocotyl.
Causes of plant stretch:
- Leaving plants in the germination chambers too long. Trays should be moved from germination areas to growing areas before the cotyledons emerge.
- Cloudy weather. Stretch is greater during cloudy conditions. High light reduces stretch. Make sure greenhouse films have high light transmission and replace film more often in transplant houses.
- Overcrowding and shading. Overcrowding is hard to avoid with trays tight together. However, shading can be limited by having clear end walls; making sure other structures, shrubs and trees do not produce shade; and limiting shade from above and around the plants.
- High daytime temperatures and large differences between day and night temperatures. Keep houses cooler in the daytime to reduce stretch.
- Watering program (watering too much). Keep plants on the dry side to avoid stretch and avoid overwatering
- High phosphorus fertilizers. Keep phosphorus levels low in greenhouse fertilizers to avoid stretch.
Stretched watermelon transplant with elongated hypocotyl