Hints on Growing Blueberries

By: Eric J. Hanson and James F. Hancock,
Michigan State University
Dept. of Horticulture

Blueberries require fairly specific soil and climatic conditions for maximum production. The commercial industry is restricted to areas in western Michigan, primarily Allegan, Berrien, Muskegon, Ottawa and Van Buren counties, where the climate and soils are ideal. Smaller patches of blueberries are also grown throughout the southern part of the state. Information in this bulletin is primarily directed towards commercial production but will be helpful also to part-time and backyard growers.

Costs of establishing and maintaining a blueberry planting are high, but returns from well managed plantings on suitable sites can be substantial. The common row and plant spacings require between 1,000 and 1,200 plants per acre. Two-year-old blueberry plants cost between $1 and $2, so it will cost between $1,000 and $2,400 per acre for plants alone.

No crop will be harvested the first two years. Properly managed plantings will yield 400 to 800 lb./acre the third season and 1,400 to 2,000 lb by the fourth year. Full crops of 4,000 to 6,000 lb/acre are generally harvested after six to eight years, although mature plantings can yield in excess of 10,000 lb/acre under optimal conditions. Well-maintained blueberry bushes remain productive for at least 15 to 20 years.

Though wild lowbush and highbush blueberries grow throughout Michigan, cultivated highbush blueberries generally grow best below a line extending from Muskegon to the lower end of Saginaw Bay. Above this line, the growing season tends to be too short and winter temperatures too cold for highbush blueberries. They require an average growing season of 160 days, and late spring or early fall frosts can damage plants. Mid-winter temperatures of –20 to –25 degrees F, which are not uncommon in northern Lower Michigan, can severely injure or kill both flower buds and young branches. Standard highbush cultivars may be grown successfully in northern Lower Michigan only if winter and spring temperatures are moderated by lake effects, or if plants are protected by snow cover.

Even in southern Michigan, late spring frosts can severely damage flowers and reduce yields, particularly in areas outside of the influence of Lake Michigan. Avoid low-lying areas surrounded by woods or hills these "frost pockets" tend to collect cold air. Higher sites with a gently slope have greater air circulation and are less prone to spring frost damage. Air movement also suppresses certain fungal diseases and provides more comfortable picking conditions during hot weather.

Blueberries have very specific soil requirements. Blueberries have performed well in Michigan on a number of soil types, including Au-Gres, Au-Gres-Saugatuck, Covert, Houghton, Morocco, Napoleon, Newton, Pipestone-Kingsville and Thetford. All these soil types tend to be acidic and sandy with a varying organic content.

Organic Matter: The soil should be high organic matter. Organic matter increases the water-holding capacity and improves the aeration of soils. Blueberries have fibrous, shallow roots that are sensitive to soil compaction and poor drainage. During the growing season, water standing on the surface for one or two days can damage roots. Avoid planting on heavy soils that drain slowly. Soil organic matter can be increased in small plantings by incorporating peat, bark mulch or straw but this is usually impractical in larger plantings.

Soil Moisture: Ideal soils are well drained with a water table 14 to 22 inches below the surface. These soils provide a constant, moderate water supply through the season but pose few problems with standing water and waterlogging.

Even on the best sites, dry seasons frequently reduce growth and yields. Many Michigan growers now irrigate to avoid drought stress. Both overhead sprinklers and trickle irrigation systems are used. The costs of establishment and potential benefits of irrigation are difficult to compare, but plantings on sandy soils low in organic matter will benefit most from irrigation. Overhead systems provide added benefit of frost protection, which trickle systems cannot provide.

Soil pH: Blueberries require acid soil with pH below 5.5, and they do best where the pH is between 4.5 and 5.0. Test the soil pH before planting blueberries. (Consult your County Extension Service for instructions.)

Highbush blueberry varieties vary in growth habit, disease resistance, berry characteristics, harvest season and yield potential. Choose varieties suited to your region and intended use. Varieties adapted to Michigan conditions are described in detail in Extension bulletin E-1456, "Highbush Blueberry Varieties in Michigan."

Although highbush blueberry varieties are generally self-fruitful, using two or more varieties that ripen at different times will lengthen the harvest season. A long harvest season may be particularly helpful if you’re growing fruit for the pick-your-own market. Harvest season and other important traits of varieties that perform well in Michigan are listed in Table 1.

Northland and Patriot are hardy crosses between highbush varieties and lowbush blueberries. They are "half-high" types that perform well in colder regions where snow cover provides protection during winter. Though the buds and wood of these varieties may tolerate midwinter temperatures as low as –30 degrees F, they have several drawbacks. Patriot blooms very early and is often damaged by spring frosts. Northland requires extensive pruning, and its fruits are small and soft. The University of Minnesota has recently released several promising half-high cultivars, but they have not been extensively tested and cannot be recommended yet for commercial use.

Test the soil and adjust soil pH to 4.5-5.0 before planting. If the pH is below 4.0, incorporate finely ground dolomitic limestone, following rates recommended on a soil test report (usually 1 to 2 tons per acre).

If the pH is above 5.1, acidify the soil with elemental sulfur. It will require 450 to 900 lb sulfur per acre to lower the pH one unit, e.g., 5.5 to 4.5. Use the higher rate on heavier loam soils high in organic matter and the lower rate on light sandy soils low in organic content. It is usually not economically practical to acidify organic soils with a pH higher than 5.5 or 6.0. If the pH in established plantings is slightly over 5.0, continued use of ammonium sulfate fertilizer will gradually reduce it.

Set plants in the fall or early spring as soon as the soil can be worked. Space plants 4 to 5 feet apart in the row on less fertile mineral soil or 5 to 6 feet apart on organic soil. Rows should be spaced to accommodate equipment, usually 10 to 12 feet apart. Set plants 1 to 2 inches deeper than they were grown in the nursery. Plants may establish more quickly on sandy soil if a shovel full of acid peat is mixed in each planting hole.

Overhead sprinklers are the most common method of irrigation, but some growers use trickle irrigation systems. Overhead systems provide the added benefit of frost protection.

Apply enough water during each irrigation to moisten the top 12 to 16 inches of soil at a rate slow enough to allow water to drain into the soil without puddling on the surface. Extension bulletin E-1110, "Irrigation Scheduling for Field Crops and Vegetables," provides more detailed information on irrigation scheduling.

Apply fertilizer in a broad strip on both sides of plants just before budbreak. Use split applications on sandy sites—apply half the rate just before budbreak and half at petal fall.

Potassium (K): Michigan blueberry soils are rarely deficient in potassium. Apply potassium if the soil test level is below 100 lb K/acre (Table 3) or if leaf analysis indicates a need.

Potassium can be applied as a 2:1:1 complete fertilizer that supplies the recommended amount of nitrogen or as Sul-Po-Mag (22 percent K₂O) or potassium sulfate (50 percent K₂O). Avoid muriate of potash, which contains chloride and can be toxic to blueberries.

Phosphorus (P): A phosphorus deficiency has not been reported in Michigan blueberries. Use superphosphate (46 percent P₂0₅) before planting to bring the soil test level to 100 lb P/acre if soil analyses indicate a need.

Calcium (Ca): Calcium deficiencies have not been reported. If leaf analysis indicates a shortage and pH is below 4.5, apply 500 to 1,000 lb/acre dolomitic lime.

Magnesium (Mg): Magnesium deficiencies occur occasionally in Michigan blueberries. If soil tests or leaf analyses indicate a Mg shortage, apply magnesium as dolomitic lime at 500 to 1,000 lb/acre if pH is below 4.5, or as magnesium oxide (55 percent Mg), Epsom salts (18 percent Mg) or Sul-Po-Mag (11 percent Mg) if pH is above 4.5.

Micronutrients: Micronutrient deficiencies are uncommon in Michigan blueberries when pH is in the proper range (4.5-5.0).

Blueberry bushes are best pruned when dormant, in either late fall, winter or early spring. Spring pruning is usually preferred because canes injured during the winter can be identified and removed. Bushes pruned before leaves fully drop in the fall may bloom a day or two later than bushes pruned in the spring.

Pruning Mature Bushes:
Mature bushes should be pruned annually. Follow these general steps:

1. Remove branches injured by winter weather or machinery and diseased canes.
2. Remove fruiting branches that are spindly or less than 1 foot from the ground. Fruits at ground level are difficult to harvest.
3. Remove one or two of the oldest canes and any branches that are low in vigor. (Low vigor canes produce thin, twiggy growth with few flower buds)

Pruning should be used to manage bush size and shape to facilitate picking, fertilizing and spraying. Blueberry twigs require at least 15 percent of full sunlight to initiate flower buds, so varieties that develop thick upright growth need to be opened to sunlight. Several of the large canes on the interior of dense, upright bushes of varieties such as Jersey and Rubel usually need to be removed (Fig. 2A and B). Varieties that have a sprawling, open growth habit, such as Berkeley and Bluecrop, should be pruned to keep the bush shape more upright and compact (Fig. 3A and B). Varieties such as Blueray and Northland may produce too many new canes each year. (Fig. 4A) and become overcrowded if they are not thinned (Fig. 4B).

Bushes harvested by pick-your own (PYO) should be pruned so that fruits are easy to see and reach. Keep the bush interior open. Remove tall or difficult to reach canes or head each back to a vigorous side branch. Remember that heavily pruned bushes compensate for fewer fruits by producing larger berries. Larger fruits are usually more desirable for fresh or PYO marketing.

Blueberries require bees for pollination and fruit set. Though native bees may adequately pollinate small plantings, most commercial growers place honeybee hives in plantings for optimum fruit-set. During average years, two large hives per acre appear to be adequate, though benefits can be gained from as many as four hives.

Bees do not work effectively during cool (cooler than 55 degrees F), windy or rainy weather. If these conditions occur during bloom, applications of gibberellic acid (commercial product "Pro-Gibb) made at late bloom according to label instructions may increase fruit-set. Sprays have little effect during years when pollination conditions are good.

Avoid using herbicides the planting year—they may injure young plants. Control weeds emerging the first year by cultivating. Avoid cultivating deeper than 1 to 2 inches—deeper cultivation may injure shallow blueberry roots.

Weeds growing within rows can be controlled with herbicides and instructions for use are updated annually in Extension bulletin E-154, "Michigan Fruit Management Guide"

To pick by hand, gently roll berries between your thumb and forefinger, removing fully ripe berries and leaving unripe berries for the next picking. A picker can collect berries in an open container attached to a belt or cord at waist level. This frees both hands for picking.

About half of Michigan blueberries are harvested mechanically using large over-row harvesters. Harvesters vibrate berries off of bushes and onto conveyors. Mechanical harvesters are usually used for berries to be processed.