INTRODUCTION

If left unattended, the nature of the wild blueberry plant is to bear less and less fruit with age. From single sprouts with plenty of floral buds, the plant will become increasingly branched and all its energy will be devoted to vegetative (green) growth, to the detriment of floral bud formation. When this happens, the floral buds will be located at the end of the stems and will be smaller, more vulnerable to winter damage and more difficult to harvest.

By observing the behaviour of wild blueberry plants after accidental pruning (forest fires), our native peoples and colonial forefathers quickly understood that to maintain the productivity of wild blueberry stands, this activity would need to be repeated on a cyclical basis. The first management of wild blueberry fields was borne of this observation. Based on a 2 or 3 year cycle, the burning of fields replaces older, branched bushes having fewer floral buds with single more productive shoots. Pruning techniques originated with the use of fire to destroy the aerial portion of the plant. In recent years, pruning by mowing has emerged as a cheaper method of pruning, and is becoming increasingly popular.

Regardless of the method used, the goal of pruning is to kill or remove the existing plants to a level as close as possible to the ground (less than an inch). If not, regrowth will arise along the length of the stem, rather than from the underground stem (referred to botanically as the rhizome).

WHEN TO PRUNE

Among growers, theories abound as to when is the best time to prune, late fall or early spring. An autumn pruning is thought to decrease the debris and give the plants a quicker start the next spring. It is definitely advisable for fields which are wet or hard to reach in the spring.

One thing is certain, pruning should be conducted during a period of dormancy, either after the first severe autumn frost (when the leaves have turned red) or before regrowth (bud break) in the spring. If pruning is conducted too early in the fall, it may result in inadequate storage of the food reserves in the rhizomes, where they are stored until the following spring. If pruning is conducted too late in the spring, this will reduce the time in which the plant can grow in height before setting fruit buds (mid to late summer). This, in turn, results in a lowered yield potential.

The method of pruning may have some impact on the most appropriate time to prune. Straw burning will be far more uniform after the winter snow has compacted the straw. With oil burners, there should be no difference, though research shows that more nutrients are liberated to the plant following a spring burn than following a fall burn.

With flail mowing, some differences have been observed. With an autumn mow, the majority of sprouts which are not cut exactly at ground level may be destroyed during the winter. After a spring mow, the aerial portions which have not been killed by the winter will break out in lateral branches, which are less productive. A spring mow should therefore be performed more exactingly.

BURNING

Burning offers several advantages over mowing. The heat used to burn the aerial plant parts will encourage the new sprouts to grow and has been shown to liberate nutrients for the growing plants. This heat also sanitizes the field by killing insects, disease organisms and weed seeds which overwinter near the soil surface. The insects controlled in this way are the blueberry spanworm, the blueberry flea beetle and the blueberry sawfly. The heat is also thought to be responsible for lowering the incidence of Monilinia blight and blossom blight (Botrytis).

A very important component of burning is the intensity of the burn. If the heat source is too weak, the burn and the subsequent crop will not be uniform. Conversely, if the heat source is too strong, the pruning and sanitation will be thorough, but this will be more costly and the organic matter layer will suffer. This layer acts a buffer area in which the rhizomes thrive, and is also thought to act as the long term nutrient source for the plants. Proper burning requires a good knowledge of the machinery under varying conditions of moisture, wind and travelling speed.

A wild blueberry stand needs to be burned only enough for the buds to pop, though stands are often burned harder than this. It is best to burn when the ground is still frozen or at least moist. This will help conserve organic matter. As a result of this, late spring burns should be light.

Burning should never be done without a permit nor without a fire-break. The rules pertaining to burning are explained in factsheet A.10.0.

The principal methods of burning are the following:

Free burn:

This method, which is only used on new fields, uses the plants and weeds (especially grasses) as the source of combustion for the fire. Though the method is cheap, the fire may be very uneven, and will tend to range from overly intense in some areas to inadequate in others. A free burn should be used on a relatively dry day with a mild wind, and a fire break is extremely important. This is an inferior method of burning, and may require touching up with a portable burner or straw.

Straw burn:

Some growers use this method of burning, particularly when they have access to an abundant and cheap supply of straw. The straw can be spread manually or by a mechanical spreader. For a quality burn, the straw should be free of weeds and spread uniformly. The best results are obtained when the straw is spread in the fall after the leaves have turned red or fallen, as the snow will settle the straw closer to the plants. One to two tons of straw may be required on an acre, depending on the uniformity of the spread, with less being required for a uniform spread.

It takes one person a day to spread, by hand, the 40 or 50 bales of straw required to cover an acre of land. A mechanical spreader can cover eighteen to twenty acres in a day. The order of preference among growers appears to be: oat straw, wheat straw, barley straw and old hay.

To get a good spread for the fire, the straw should be a foot in length. Greater quantities are required if the straw is short. Moist and clumpy straw should be avoided. The ideal time to burn with straw is in the spring when the ground is fairly dry, with a temperature close to 20 C, on a day with a light wind. Excellent fire-breaks are necessary, and burning should be done in a piece-meal fashion, starting with the section of the field which is the furthest downwind.

Straw burning is best suited to fields that are too steep and/or too rocky for pruning with machinery.

Oil burning:

The first oil burners were developed in the 1940's. They had 3 or 4 heads like torches. Oil consumption with these burners were of the order of 227 litres per hour. During the energy crisis of the 70's, more economical burning heads were developed, including the Bossé heads and economy heads. These improvements resulted in savings of 40 to 60% on energy expenditures, compared to conventional heads. A study conducted in Maine demonstrated the following variations in oil consumption for the three types of burners:

• conventional heads: 459 litres/ha (41.2 gal/acre)
• economy heads: 277 litres/ha (24.7 gal/acre)
• Bossé heads: 185 litres/ha (16.5 gal/acre)

When pruning with oil-burners, it is very important to have fire-breaks around the field. In addition, growers should expand the line of protection by burning a band on the perimeter of the field. This is especially important if there is a lot of debris in the field.

The preferred weather for burning is nice, sunny weather. Rainy days should be avoided. Unfavourable burning conditions can increase oil consumption by 30 to 40%.

MOWING

The flail mower is the latest technology for pruning wild blueberry fields. Mowing can be as good as burning if it is well done, since it acts in the same fashion by destroying the stems and encouraging regrowth from the rhizomes. In fields with low-lying surface rock, only this type of mower should be used, since it is the only type that can endure this type of abuse. Rotary mowers (brush cutters) are not recommended, because they cause too much damage to the land and do not produce a uniform cut. While it is possible to use a lawn mower, the technology can only be used on very small properties without shrubs or rocks.

The advantages of flail mowing:

• Stems which are cut at soil level and chopped by the mower should reduce the hindrance to harvest and improve the quality of the harvest.
• The plant residues left on the soil surface may help to retain moisture.
• The presence of residues may help to reduce soil erosion in susceptible fields.
• The technology can help to maintain and improve organic matter which can reduce pesticide leaching.
• An improvement in organic matter maintenance and therefore plant nutrition.
• An improvement in organic matter at the soil surface can help to regulate soil temperature, thereby allowing the rhizomes to spread better.
• Weather conditions have much less influence on the timing of operations for the use of flail mowers than for the use of burners.
• In fields with excellent weed control, it is not necessary to use brushcutters before flail mowing, thereby resulting in savings. Depending on the type of mower and the condition of the blades, flail mowers may perform well, even on brush that is as much as 2.5 cm. (1 inch) in diameter.
• The cost of flail mowing can be as low as one third that of burning, thereby resulting in greater profitability.

The disadvantages of flail mowing:

• The terrain should be relatively flat and free of rocks, and this can incur large land improvement costs.
• The incidence of diseases and insects can be much greater using this technology.
• If the technology has not been mastered properly, mowing can result in decreased yields from the presence of short, branched and unproductive plants arising from stems which have been cut too high.

Types of flail mowers:

Several companies sell varying capacities and qualities of flail mowers, some of which have been designed specifically for wild blueberry fields. For rough terrain, it is important to seek out models which have shafts and blades that are sufficiently solid. Flail mowers vary in width from 60 cm to 4.5 m, and their use is determined by the condition of the terrain. Some are powered by the tractor, others by hydraulics and others have their own motors. Mowers can be gang-mounted to cover a greater width, while maintaining the ability to follow the contours of the terrain. The choice of a mower should be given a lot of consideration, and chosen in relation to the field and the working conditions.

Getting the most from flail mowing:

• Select a width which is appropriate for the field. A narrow mower will follow the lay of the land more easily.
• Wide mowers will tend to shave the knolls and to miss the hollows.
• It seems to be a good idea to fertilize fields in the spring, after mowing. This is thought to be due to the fact that nutrients are not being released as they are in a burned field, and the fact that the decomposing plant tissues may be "robbing" nutrients (especially nitrogen) from the soil.

Tractor speeds:

• Proper mowing requires a slow travelling speed. The normal speed is in between 1 and 2 mph. For a fall mow, the plant does not need to be fully cut, since removal of the outer bark is sufficient. Winter conditions will destroy the stem.
• The height of the cut should be verified periodically and adjusted according to the wear on the blades. It is important to cut as close to the soil as possible. If the cut is too high, there will be an outgrowth of side branches on the stem, and these side buds tend to be less productive. Mowing should not be so low as to damage the rhizomes.
• Mowing should be done after the leaves have turned red, and can continue until snowfall. Fall mowing reduces the incidence of regrowth from poorly cut stems, since the winter will help in removing and destroying these.
• It may be necessary to perform some spot-burning in those areas of the field where access to mowers is tricky or impossible.
• A matter of safety: Keep your distance from a working mower and stop the tractor if it is necessary to do work on the mower.

THE FUTURE OF PRUNING WILDBLUEBERRY FIELDS

Economics and other factors will probably dictate an increase in the use of flail mowers. Burning will probably be retained as a periodic treatment for field sanitation or combinations of mowing and light burning may evolve. Other forms of sanitizing fields are being studied, including aspirators which burn debris and return it to the field. Several companies are also working on superior methods of burning with propane.

CONCLUSION

Pruning techniques can be considered equivalent, as long as the plants are dormant and the techniques are used well. Success of any technique will depend more on its suitability within the field's cropping system than on the technique itself.

Certain segments of this document were adapted from factsheet # 229, "Pruning Lowbush Blueberry Fields, University of Maine.

Prepared by Gaétan Chiasson, P.Ag. Horticulturlal Specialist , and John Argall, P.Ag., Blueberry Specialist, N.B. Department of Agriculture & Rural Development
Fall 1995