CO-Horts Blog

Monday, February 16, 2015

Warm Weather and Winter's Return

Posted by Eric Hammond- Colorado State University Extension Adams County
“Gosh it’s just so beautiful out today”

“Can you believe the weather?  We are so lucky we live in Colorado!”

“It’s flip-flop weather, I love it!”

These are the kinds of statements I have overheard over the past few weeks while the high temperatures have been in the 60’s and 70’s.  As much as I have enjoyed going jacketless, I have to confess these sorts of comments are starting to make me cringe.   
This heat wave started in early February (yes I used bold, underline and italics).  Sure, it’s not uncommon for us to experience a warm day here or there in the winter.  It’s one of the perks of living in Colorado.   However, this has been two solid weeks of warm weather.  Even the mid-week storms we have had were short lived “warm” storms.   These consistently warm mid-winter temperatures come with significant risk for landscape plants.

Some of the effects of all this abnormally warm weather are easy to see.  For example, many spring bulbs are already coming up.  Daffodils, crocuses and even some tulips are already peaking up, especially on the west sides of structures.  Some cool season weeds like redstem filaree and cheat grass (downy brome) have germinated are actively growing and consuming water and nutrients from the soil.   Beyond landscapes many winter annual crops such as winter wheat, encouraged by the warm weather and warming soil temperatures have been actively growing and may now be at risk if we receive a sudden hard frost.

Many spring bulbs have already sprouted

What worries me more is the changes in plants we can’t see that are brought on by the warm weather, especially in woody plants.  These plants go through a complicated and gradual process to acquire hardiness in the winter.  The main goal of which is to avoid the formation of ice crystals within their cells (this process is fascinating in a science-geek sort of way and more information on it can be found here:    This process is reversed by consistent warm temperatures leaving woody plants at risk if temperatures suddenly drop again.   

If woody plants have lost of some hardiness during the last couple weeks we might see several things if temperatures should suddenly drop:

·         The death of small outer branches of trees.  These branches are more exposed to cold air and do not have the thermal mass of larger branches to buffer them against temperature changes.

·         Frost cracks in trunks and larger branches.  Cambial tissues which have lost hardiness or which have damaged tissues from a previous cold event (like the early November freeze we had last year) are prone to frost cracking.

Arborvitae damaged by sudden cold temperatures last November.  Many woody plants were damaged by this event
So what can we do as gardeners and landscape managers to deal with fluctuating temperatures?

·         Use organic mulches such as woods chips or straw.  Organic mulch acts as an isolator which helps to mediate soil temperatures.   During the day, soil under an organic mulch does not get as hot (it’s insulated from solar radiation) and during the night the soil may not get as cold (it’s insulated from cold air).  Keeping the soil cooler during the day may help prevent bulbs from prematurely sprouting and help trees maintain cold hardiness.

·         Avoid pruning until early spring.  Pruning cuts can lead to cracking when exposed to extremely cold temperatures or rapid temperature fluctuations.

·         Wrap the trunks of young trees to help insulate them against temperature changes.  Trees should generally be wrapped around Thanksgiving and unwrapped at Easter.  Paper wraps are preferred to plastic.  More information can be found here: and

·         Protect spring bulbs which have sprouted with a thick layer of organic mulch (cover the plants).  Mulch can help insulate the tender shoots of bulbs from cold temperatures, to a point.  Be sure to remove the mulch in the spring, as deep mulch can restrict gas exchange and lead to low soil oxygen levels. 

·         Winter water when appropriate.  Plants that are healthy better tolerate most environmental stresses.  More information on winter watering can be found here:
Winter watering should be done about once a month if
we don't have significant snowfall

·         Follow other best management practices to improve plant health.  Plants that are healthy and have higher energy reserves are better able acclimate to changing temperatures.

·         Use appropriate plant material and site more tender material in sheltered sites.

·         Don’t panic.  No one can control the weather.

It does not look like temperatures are going to drop quite as severely tonight as was forecasted last week (at one point the low for Tuesday morning was forecasted to be just north of 0 degrees Fahrenheit) and I’m not saying that next time your co-worker, significant other or neighbor gushes about the warm weather you should lecture them on cold hardiness in plants, or normal weather for February in Colorado.   However, you might chime in with a word of sympathy for the plants or remind them to winter water.  Colorado is a tough place to be a plant.


  1. With the freeze last November and with the roller coaster temps we are experiencing's tough being a plant/tree in Colorado. Excellent tips Eric.

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