Citrus and Freezing Weather

I am going to give you my suggestions on how to get your citrus through freezing weather.  To be on the safe side, you ought to keep your citrus above 26°F.  This is a conservative temperature and some citrus can survive much colder temperatures.  Tender new growth and fruit will freeze at slightly warmer temperatures.  Duration is also important because the plant or fruit has to have time to lose its heat before it is damaged.  Thick skinned fruit needs more time to lose its heat.  Of course, leaves and thin branches lose their heat the fastest so it will be damaged first. 

What happens is ice forms in the plant cells and ruptures the cells.  Now the state of the plant tissue will also determine its ability to withstand freezing temperatures.  If the water content is low in the plant cells and contains sugars or other dissolved plant chemicals, the temperature where ice crystals form will be lower (similar to the adding of salt to the ice water mixture to make old fashion ice cream).  Dissolved salts make the freezing temperature lower.  This is accomplished to a small degree by grafting varieties on to trifoliate rootstock which almost cuts off the water supply to the grafted scion.

My Fairchild mandarin Christmas day 2004.

Most Hardy to least Hardy:
 
An approximate list of the most hardy citrus trees to the most sensitive citrus trees:  trifoliata, trifoliata hybrids, citrangquat, kumquats, kumquat hybrids, yuzu, satsumas, meyer lemon, mandarins/tangerines, oranges, tangelos, pummelos, grapefruit, lemons, limes, citrons.

Suggestions for freeze protection for trees in the ground:

• Provide a wind break, if possible (plant on the south side of a building).  A stone or brick wall will absorb heat in the day and give it up at night.
  
  
• Cover the tree to trap heat and prevent radiation to a clear sky.  This could be black plastic (the leaves touching the plastic will be burned), sheets, tarps, or better blankets.  Of course, wind is going to be a problem so if possible, anchor down with bricks, lumber, buckets of water, etc.  It is better to cover in the shape of a tent to capture ground heat than to cover like a mummy.
  
  
• Put a trouble shooting light, old fashion Christmas tree lights if you can find some (the big ones not the LED type), or a cheap electric bathroom heater set at its lowest temperature and wired to the tree so it will not fall over (about $20 to $30) under the cover.  Be sure and turn off the heater if rain is going to occur.
  
  
• Put a large trash can/s full of water next to the tree and under the cover.  The freezing water will give off heat.
  
  
• Water the ground around the tree the day or evening before.  But don't wet the leaves if possible.  Wet leaves in wind will encourage freezing evaporation, but freezing water on the ground below the tree releases heat to the tree.
  
  
• Do not put mulch or leaves under the tree.  This will blanket the soil and not let the heat rise from the soil under the tree.
  
  
• Surround the tree with chicken wire and fill the whole circle with leaves covering the tree.
  
  
• Bank dirt up to cover the graft on the tree to act like an insulator and save the graft.  Although, this will not help save the branches and leaves.
  
  
• Build a box around your tree or if you tree is small put a large trash can or large cardboard box (furniture, refrigerator, etc) over the tree.
  
  
• Put pipe insulation on the branches.
  
  
• For large trees, about the only thing you can do is put a micro sprinkler under the base of the tree and mist up to the lower branches.  The sprinkler has to be started before freezing temperatures and end after temperatures get above freezing.  Hopefully, this will save at least the graft.