Understanding Frost & Freeze Injury: Tight Cluster to Bloom

Growers throughout Illinois, Iowa, Minnesota and Wisconsin are reporting McIntosh, Zestar and other varieties at tight cluster.  We clearly have an early start to the season and without being alarmist, it is beneficial to revisit the topic of frost injury to fruit crops.

On April 21, overnight temperatures of 27 ºF were reached throughout Wisconsin and at the tight-cluster stage, this does pose risk for frost injury to the crop.  Prior to these sub-freezing temperatures, average-low temperatures were in the upper 30s ºF and low 40s ºF. Nighttime temperatures are to remain in the mid to upper 30s ºF for the remainder of the week and not climb back into the 40s ºF until the weekend. The 30 day forecast for southern Wisconsin does not predict freezing temperatures during the month of May.  However, these long-term forecasts can change.

Frost or freeze injury does not occur simply during the presence of cold temperatures.  Injury is caused to plants by the formation of ice inside the plant tissue.  Under these conditions, water is drawn out of the plant cells, causing dehydration and ice is formed around the plants cells.

During extended cool periods,  plants can harden off and become resilient to freeze events, if temperatures are not too cold.  When temperatures warm up, this hardening off is lost. This should not be interpreted as absolutely correct, but could be helpful to understand the current situation.  We have had some warm days, but these have not been sustained.  The nights have still been cool and we had several days of cool weather prior to temperatures that declined into the upper 20s ºF.

Below are links to two publications from the Food and Agricultural Organization of the United Nations, which discuss the science behind different freeze events and opportunities to use different frost-protection strategies to mitigate crop losses.


FAO: Frost Protection, Volume 1

Page 82. Physiology and critical temperatures for frost injury on fruit trees
Page 85. Critical temperature values for apples (ºC)
Page 130. Chemicals to delay bloom
Page 143. Active frost protection

  1. Heaters
  2. Wind machines
  3. Helicopters
  4. Sprinklers
  5. Surface irrigation
  6. Foam insulation
  7. Foggers
  8. Combinations of active methods

FAO: Frost Protection, Volume 2