Tree Freeze Alert Issued for Palo Alto

A "tree freeze alert" has been issued by Canopy for Palo Alto as a frigid cold front bears down on the region.

Residents are warned to take preventative action to protect vulnerable trees, which include citrus, jacaranda, catalpa, oleander, eugenia and other tropical/sub-tropical plants, according to the Thursday alert from Canopy, a nonprofit that "grows the urban forest" in Palo Alto and East Palo Alto.

Steady rain is expected to fall starting Thursday evening and through Friday night, and may turn into snow by Saturday, according to the National Weather Service.

Canopy Executive Director Catherine Martinaeu said that although frosts are relatively common during the winter, this one is coming dangerously late in the season.

“Some trees are starting to put their leaves out, and usually the trees are better adapted to frost when they don’t have their leaves out,” she said, adding that farmers in particular have cause for concern. “In terms of fruit trees, it could certainly compromise the harvest.”

Temperatures are expected to drop into the 20s this weekend, which can damage tree buds, blossoms, fruit, leaves and twigs, according to Canopy.

The threat posed by the coming weather system was first noticed by certified master arborist Dave Muffly, a Canopy board member. Mountain View Trees also issued a tree freeze alert for Mountain View.

Canopy recommends that tree owners use burlap, sheets, tarps or similar fabrics to cover their plants, and that those covers extend all the way to the ground in order to retain ground heat.

It is also recommended that covers are separated from the foliage with a frame or stakes. Below are other practical tips offered by Canopy, all of which appeared in the alert sent earlier today.

Keep plants well-watered.

• Moist soil will absorb more solar radiation than dry soil and will re-radiate heat during the night.
• If you have a large tree that needs protection, running sprinklers at the coldest time of the night (usually from 4-6 a.m.) can give it a slight edge. The strategy makes use of latent heat released when water changes from liquid to a solid.
• When ice crystals form on the leaf surface, they draw moisture from the leaf tissue. The damage from this dehydration will be less severe if the plant is not already drought-stressed.

Plan ahead.  

• Remove turf/weeds from under trees' canopies, as bare soil absorbs and reflects heat best.
• Wood chip mulch prevents soil moisture loss and insulates roots.
• Plant frost-sensitive plants near sources of reflective heat (like buildings, walls, etc).

After the frost, help trees recover. 

• Do not prune anything off immediately. Wait to see what sprouts in the spring; the damage is often not nearly as bad as it initially looks, and new growth may come out of tissue that you thought was dead.
• If dieback is severe enough and your tree has lost "shade," protect the now-unshaded portions of the trunk/branches from the sun with a physical cover or with whitewash (1:1 ratio of latex paint and water).
• Remove frosted/mushy fruit while still salvageable for snacking on or juicing.
• Wait to fertilize (if necessary) until the cold weather has passed. There is no evidence to indicate that frozen trees respond to any special fertilizer that is meant to stimulate growth.