Winter Injury On Blueberries

By Heather Bryant, Regional Field Specialist, Food and Agriculture

This time of year blueberry plants are dormant, and this is the perfect time to start thinking about pruning.  If like me you are looking out at a field covered in snow, don’t worry you have until the plants start growing again in the spring to get it done.  Meanwhile, pruning season is a good time to take stock of what has and is happening this winter and start planning for the upcoming season.

I’m writing this during a heavy snowstorm, but actually we made it through most of December and January without much snow on the ground.  In addition to that it has been extremely cold.  Unfortunately this is not ideal weather for blueberries or fruit crops in general.  One of the concerns to look for this coming season is winter injury.

Blueberries can be adversely affected by four different types of cold temperature stress.  The first issue is cold winter temperatures.  Even the cold hardy blueberry varieties we would recommend for this area can be damaged at temperatures below negative 25°F.  Depending on where you are, some of you may have seen temperatures that cold this winter.  If you haven’t planted yet, you can guard against this to a degree if you choose a site where the cold air can drain away from the plants and if you plant cold hardy varieties.

The second and somewhat counterintuitive cause is moisture stress.  In the winter soil moisture is frozen and the air tends to be dry.  Add wind, and the buds and plant tissue can become desiccated, especially if the plants are not insulated by snow cover.  Choosing shorter stature varieties and pruning your plants shorter when they get excessively tall will help with this by keeping the buds closer to the ground.  A wind break may also help as long as it doesn’t prevent air drainage.

A third issue is temperature fluctuations.  Blueberries have a chilling requirement, once that requirement is met, if the temperatures rise the plants will start to come out of dormancy.  If the temperatures drop again the plant is now more susceptible to cold than it was when it was still dormant.  We haven’t had that many warm days, so hopefully that is not an issue this year.  Avoiding steep planting sites with southern exposure can minimize this risk.

The fourth issue and the one that is an even bigger issue for tree fruit, is the risk of a late frost just when the plants are flowering.  Blueberry flowers can be damaged by temperatures below 28°F.  Again, one way to guard against this issue is to plant on a site with good air drainage.  It’s also useful to know that flowers on small short diameter stems will open before the flowers on large diameter stems.  If frost during flowering is a common occurrence for you, prune for larger diameter stems to try to get your plants to flower as late as possible.  Other than that, we can only cross our fingers the temperatures don’t drop while the plants are flowering.

So if winter injury has occurred, what are the symptoms to watch for?  In most cases the damage won’t be obvious right away.  The tips are more susceptible than the lower sections, so you may see some canes with dead tips.  This tends to be even more noticeable in raspberries.  The plants will also sometimes leaf out in the spring and appear fine, and then all of a sudden some of the canes just seem to collapse.  While there are other things that can cause this, winter injury is a common cause.   It means that the roots were damaged but not killed.  They had enough vigor to get the plants started, but as the leaves matured and the berries started to form, the plant began to demand more resources than the weakened roots could provide.  Proper pruning aimed at not allowing the plants to become over grown may help with this.  

If you’d like to learn more about caring for blueberries and other fruit crops, Cooperative Extension runs a series of pruning demonstrations across the state each spring.  For more information on demonstrations near you please visit http://extension.unh.edu/resources/files/Resource003681_Rep5246.pdf

Source : unh.edu