Cool Greenhouses and Vegetable Transplants

March 6, 2014

Many growers are considering whether they should lower their greenhouse thermostats this spring to reduce fuel costs.  Today, temperatures in Massachusetts range from 15°F to 27°F across the state.  No doubt about it, we’ve had a cold winter. Temperature affects many aspects of production and establishing the best conditions for your greenhouse transplants will ensure a good start to the season. Here are some tips:

  • Avoid growing transplants near the ground as ground temperatures can be 10 to 20°F cooler than air temperatures (unless bottom heated). Before starting crops and transplants in the greenhouse, place thermometers at the height where your crops will be growing.  Both soil and air (high/low) thermometers can help determine best placement for your transplants.
  • Start your crops at their optimum temperature to ensure good germination and rooting for 2-3 weeks before reducing greenhouse temperatures.  (For example, celery seeds germinate best at 70-75°F, and temperatures should be maintained above 55°F to keep celery from bolting and above 50°F to keep broccoli and cauliflower from heading prematurely.) Optimum germination and growth temperatures of vegetable transplants can be found in the 2014 New England Vegetable Management Guide.
  • To maintain short transplants, minimize the difference between day and nighttime temperatures. The greater the temperature difference, the more plants stems will elongate. When the day temperature is very warm and the night temperature is cool or cold, plants will be taller.
  • Cold soil and media temperatures increase the risk of root rots caused by Pythium, Rhizoctonia and Thielaviopsis. Cold also reduces crop uptake of water and nutrients, so, condensation and humidity levels will remain higher, leading to a greater risk for diseases like Botrytis on leaves. Water transplants or apply fungicides at the warmest temperatures in the greenhouse this time of year.
  • Although production time is longer, crop quality for some crops may actually increase at lower temperatures. Cool average daily greenhouse temperatures (50 to 60F) can be used to slow growth and prepare transplants for outdoor conditions such as wind and higher light levels.

Katey Campbell-Nelson, UMass Extension Vegetable Program and Tina Smith, UMass Extension

Fact sheet (Ornamental Spring Crops): Growing-on at Cooler than Optimum Temperature