Friday, May 8, 2009

Blueberries and the Weather: what do they like, what do they need?

When all else fails, talk about the weather. In this case, weather is a very relevant topic. What type of weather do blueberries like, what do they need, what can they do without.

In the winter, blueberry plants need a certain number of chill hours. The way I understand it is that a chill hour is any hour that the temperature is below 40 degrees Fahrenheit. Different varieties have different chill requirements, some of the Florida varieties needing 200 or less and others in colder climates needing many more than that.

In North Carolina, we also like to get our rainfall numbers up in the winter because we’ve seen unseasonably dry spring, summer and fall in years when tropical storms and/or hurricanes don’t make landfall in our area.

In the dormant stage, blueberries are hardy and can handle cold weather pretty well. The problem comes when they’ve reached the minimum number of chill hours required for coming out of dormancy. After this point, if the weather fluctuates too much, this causes early bloom that can’t be sustained to fully grown blueberries because of the near certainty of later frost and freeze.

To help alleviate frost or freeze damage, many farms use overhead sprinkler irrigation. When the temperature is predicted to go below freezing, the water is turned on and pumped onto the blueberry fields. The way I understand it is that this water forms an ice ring around the small blueberries on the plants. Something about the friction of the sprayed water and the ice cause the berry inside to be insulated against further freezing because between the ice and the berry is a layer of water or air. Someone feel free to correct me if I haven’t explained this correctly.

A cold and/or rainy season when blueberries are flowering causes reduced blueberry conversion from flowers or bloom because the bees are less effective in rainy or cold weather. The flip side of this is that you don’t want it to be too warm too early because in these cases, the production is accelerated and then wiped out by a late frost or freeze as described above.

Rain right up until the day before harvest is a good thing, but once you start picking blueberries, you’d rather not have rain on the actual ripening fruit if you can help it. This is especially true if the harvesting gets behind schedule. Ripe or overripe fruit and rain do not mix well.

Also, with rain during the harvest season, there is diminished fruit flavor for a period of time after the rain. The blueberries drink up lots of water and until they can convert this water into berry material, the berries can get a waterlogged taste. We’ve had customers say that they rushed right over to get the big rain berries but came out of the field with disappointment because the size didn’t compensate for the diminished flavor.

I’d say the dog days of summer are when the humidity is 100% and the temperature is also hovering around 100° and it has to rain every afternoon just to keep us from drowning in waterlogged air. This typically happens in July and August and comes after most of the blueberry production is over, at least in coastal North Carolina, which means we don’t have to worry about it affecting the harvest season. This is also the season for tropical storms and hurricanes that can damage or uproot plants and cause flooding.

Hurricane season starts in June and does not mix well with the harvesting of blueberries. We had the last 1/3 of our season completely wiped out by a small tropical storm a couple of years ago due to the wind stripping the berries right off the plants and effectively eliminating the last 12 days of the season.

Summer and fall are times for plant growth in a blueberry field. Directly after harvest, we cut back the plants and fertilize them to stimulate growth until dormancy. Rain is nice in this season and if there isn’t sufficient rainfall now or any time during the year, irrigation is used to supplement natural rainfall.

At Farmer Mac’s Berries, we use drip irrigation from well water. The drip tape snakes through the plants on each row, causing drops of water to fall every 6 inches or so down the row. The beauty of drip irrigation is that it is very efficient and there is significantly reduced evaporation of the water into the atmosphere, so the plant gets more of the water that is pumped onto the field than with any other method of irrigation.


  1. My question is will they survive tropical weather?

  2. My question is whats fine day to pick bluberries?