Fact Sheet No. 229, UMaine Extension No. 2168
Prepared by Tom DeGomez, Extension Blueberry Specialist, The University of Maine, Orono, ME 04469. February 1988.
It is believed that Native Americans periodically burned off trees and shrubs of the sand plains of eastern Maine to stimulate blueberry production. Wild blueberry growers have found by experience and through research that periodic pruning, by fire or mowing, can stimulate higher yields. Pruning is accomplished by removing as much of the above ground portion of the plant as possible. Yields are generally highest when fields are pruned after each harvest, creating a two-year cycle, with the first year vegetative and the second year fruiting. Traditionally, fields were pruned by free-burning the blueberry plants and weeds. The hope was that the entire field would burn. Free-burning offers a very inexpensive method of pruning, yet is very hard to control and often does not burn the field completely. Spreading an even layer of straw over the field for fuel or the use of oil burners gives the grower much more control. In the late 1970′s, in an attempt to reduce the cost of pruning with straw or fuel oil, research was conducted using a flail mower to prune. Through modification, flail mowers will follow the contour of the field providing an inexpensive yet acceptable pruning method.
Whichever pruning method is chosen, it is important to remove as much of the stem as possible. An unpruned or partially pruned blueberry plant will have an excessive number of branches. Branched plants are more difficult to harvest and less productive since a greater amount of energy is put into vegetative growth. A greater percentage of fruit buds on branched plants are produced on the terminals (tips) of the branches, making them more susceptible to winter injury than buds produced nearer the base.
Time of Pruning
If pruning is delayed in the fall until after the first killing frost, most of the nutrients in the leaves will be translocated (moved) to the rhizome. Pruning may be done from the first killing frost until growth resumes in the spring. Canadian researchers (Eaton and White, 1960) found that pruning early in the fall or in the late spring, after growth has resumed, will have a detrimental effect on the number of stems, length of stems, and the total number of flower buds produced.
Pruning with fire offers many advantages that mowing does not. The heat produced by fire will not only kill the stem, but may also reduce the incidence of insects, diseases, and weeds. Insects overwintering on or near the surface of the soil may be killed by the fire’s heat. These insects include: flea beetle, leaf beetle, spanworm, and strawberry rootworm. The heat produced during pruning may also destroy overwintering fungal organisms on the soil surface, such as mummy berry, botrytis blight and powdery mildew. Pruning with fire will reduce small coniferous trees and some weeds that spread by seed. Seeds on the weed stalks and on the soil surface may be burned, especially in the fall, thereby reducing the number of plants that could be produced in following years.
When pruning with fire it is important that the intensity of the fire is controlled. Overly hot fires do a good job of pruning but can destroy valuable organic matter on the soil surface. An overly hot fire usually means that excess fuel has been used which raises the cost of pruning. On the other hand, fires that are not intense enough will not kill the stems to the ground or will skip spots. Free-burns that rely on natural fuel in the field generally will burn too hot or too cool.
Methods of Burning
Pruning with mowers is a fairly new method. In the early 1980′s research at the University of Maine demonstrated that when flail mow pruning is done properly, it is as effective as burning. Pruning with mowers costs less and will eventually increase the organic matter on the soil surface. However, fields should be monitored much more closely for disease and insects.
Pruning with mowers is just another way to remove the blueberry stem and stimulate the plant to put out new shoots from the rhizome. Flail mowers appear to be best suited for mower pruning because the configuration of the blades prevents damage to the mower in rocky conditions. Rotary mowing of small fields with a lawn-type mower is feasible if the operator is careful not to hit rocks and scalp knolls.
Advantages of mower pruning include:
As with most practices, there are possible disadvantages to mower pruning which need to be considered. These include:
The insects that may become more prevalent in flail mowed fields are: blueberry spanworm (Itame-argillacearia), blueberry flea beetle (Altica-sylvia) and blueberry sawfly (Neopareophora-litura) and two species of Pristiphora. For identification and control methods of these insects refer to the Wild Blueberry Insect Fact Sheets and the Chemical Insect Control For Wild Blueberry Fact Sheet.
Additional Hints for Successful Flail Mow Pruning
Pruning costs will vary greatly depending upon the type of equipment used and the number of acres pruned. As the acreage increases, the cost of the equipment per acre decreases. It is important to match the equipment to the work that needs to be accomplished. It would be impractical to prune 1,000 acres with a three-foot flail mower, or to use a mechanical straw spreader to spread straw on five acres of land every year.
For a small farm, sharing equipment with another grower may be a good alternative to purchasing costly equipment. It may be quite easy to work out share schedules to satisfy all the owners. For instance, flail mowers can be easily shared because mowing can begin in October and continue into December most years with many days of good pruning weather. When large acreage is involved it is often advisable to have at least one owner or a shared employee available to operate the equipment on a full-time basis. This will ensure that the group is getting the maximum use from the equipment and good operation days are not lost.
The following tables will help estimate the costs of the various pruning techniques on different size farms or cooperatives. (Actual costs on your farm may be different, these figures are for comparison only.)
|Estimated Total Pruning Costs*|
|Farm Size (acres)
|Operation Costs Per 9 Acre ($)|
|Straw burning – manual spreading||$148.89||$123.43||$119.64|
|Straw burning – mechanical spreading||211.08||98.97||85.94|
|Oil burning – conventional heads||236.31||112.74||98.68|
|Oil burning – Bosse’ heads||225.43||77.65||61.17|
|Source: Hanson, et al., 1982.
* Includes equipment, labor and material costs, based on June 1984 prices.
**10 A costs based on 16HP tractor and 1 Mott mower; 100 A costs based on 16HP tractor and 2 Mott mowers; 1,000 A costs based on 35HP tractor and 3 Mott mowers.
|Labor Requirement and Initial Investment|
|Pruning Techniques||Labor Requirements
|Initial Investment ($)|
|Straw burning – spread manually||11.30||0.00*|
|Straw burning – spread mechanically||2.95||22,925.00|
|Oil burning – conventional heads||2.43||24,800.00|
|Oil burning – Bosse’ heads||2.43||25,450.00|
|Source: Hanson et al., 1982. (prices reflect 1984 updating) *Does not include truck needed to transport straw from the stack to the field.|
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