The Basics of Blueberry CultureMarie Townsend
Of all cultivated fruits, blueberries are one of the best for the home garden. The beautiful bushes blend well into landscaping, growing four to eight feet tall with small, white urn-shaped flowers in spring followed by big crops of blue berries and lovely autumn foliage. Nutritional research has shown that blueberries provide more antioxidants for good health than any other fruit.
Related to rhododendrons, there are three basic types of blueberries. The highbush blueberry (Vaccinium corymbosum) is the type most commonly grown in home gardens. It is one of the newest fruits to come under cultivation, with selective breeding beginning in the early 1900s from eastern North American native plants, naturally growing at the edge of bogs above the edge of high water marks in a thick mat of high acid mulch. Often growing in sandy soils above hardpan, blueberries require ample moisture but are not bog plants. For success with blueberries, try to duplicate a native blueberry's natural habitat in the home garden by supplying well-drained acid soil high in organic material and adequate water.
Because blueberry plants will live many years under suitable conditions, taking time to properly prepare the planting site will bring big rewards in future berry production. Blueberries never needed to develop water and food-seeking roots in their natural setting; their roots form a shallow mat with fine fibrous roots that do not have root hairs. Roots seldom reach deeper than 14 inches or extend beyond the drip line of the bush. Most roots are found in the 6- to 10-inch level of soil. Acid-loving plants like blueberries do not secrete their own root acids to bring iron and other nutrients into solution for use. Instead such plants rely on soil organisms found in acidic soil to convert nutrients for them. Blueberries require an acidic soil with a pH between 4.5 and 5.5. Northwest soils tend to be on the acid side, but a soil test can provide exact numbers.
Clay-loam soils must be amended to provide blueberries with well-aerated soil with a high water-holding capacity. If necessary, remove soil to a depth of 14 inches by two feet wide and fill back with amended mix. The National Clonal Germplasm Repository at Corvallis where an extensive blueberry collection is maintained recommends this mix for their Vacciniums: 50% pumice (for good drainage), 40% peat moss, and 10% loam (mix 1 part loam and 4 parts peat , for each 1 part of this mixture add 1 part pumice). Canadian sphagnum peat moss has a pH of 3.0 to 4.5 making it an ideal amendment for blueberries. Be sure to add only moistened peat to the mix. Dry peat tends to float and shed water on the soil surface. If peat is unavailable or too expensive, thoroughly mix well rotted, preservative-free sawdust (never use cedar—it is toxic to blueberries), composted bark dust, or shredded leaves to create a soil with very high organic content. If necessary lower pH and supply nitrogen by adding 1 lb. of ammonium sulfate commercial fertilizer to 100 feet per inch depth of sawdust. Organic growers can use elemental sulfur and cottonseed meal or feather meal. It takes several months to a year for sulfur to lower the pH. Prepare blueberry planting site well in advance for best results.
Blueberries require good drainage. They cannot tolerate a water table higher than 14 inches below plants. Make a raised bed up to 18 inches high and 3 feet wide to create proper drainage for the blueberry site if there is a high water table. Blueberries are not bog plants and can develop root rot in saturated soil.
While highbush blueberries are self-fertile, they produce better crops with two varieties for cross pollination and to give extended picking if selected for different ripening times. Field-grown (bare root) or container-grown 2 to 3-year-old plants grow best. Plant by early spring while plants are dormant. Container plants can be planted in the fall. Bushes should be spaced from 4 to 6 feet apart with 7 to 9 feet between rows. Home growers can grow blueberries as a hedge with 2- to 3-ft. spacing, but pruning and fruit cultivation will be harder. Blueberries are brittle. Plant where they will not be disturbed by animals or playing children. Set at the same depth plants grew in container or ground. Planting too deeply will smother the roots. Little or no pruning is needed for the first two years except to remove broken or rubbing branches. Strip off all blossoms and fruits until the plant is well established (36 to 42 inches tall).
Blueberries require mulch. For the first year mulch with fir, pine, or hemlock well rotted sawdust extending out 2 feet around plant to a depth of 3 inches so roots will not grow up into the mulch. After the first year increase mulch to a depth of 6 inches mounded around the crown. As plants mature extend the mulch out to the drip line (edge of bush). Mulch will gradually break down to help feed the plant and supply the high rate of organic matter blueberries need. More important, mulch provides a cool root run and conserves moisture, both requirements for blueberries. Other materials may be used as mulch such as composted pine needles or shredded leaves. General compost made from vegetable materials may be too high in pH for blueberries. Fresh grass clippings can heat up and damage surface roots.
Blueberries should produce new shoots at least 12 inches long on older canes and should put out several new canes from the roots every year. Plants respond well to fertilizing, especially nitrogen. However, they assimilate nitrogen in the form of ammonium better than in nitrate form, which is toxic to blueberries. Ammonium sulfate (21-0-0) will provide nitrogen and keep the soil pH in the proper range. Do not fertilize for at least six weeks after planting. During the first year use liquid fish fertilizer or liquid rhododendron fertilizer. The next year beginning in March (at bud break) fertilize by pulling back the mulch and broadcasting 4 ounces (1/2 cup) ammonium sulfate to the drip line of each bush or stir into mulch. Every year increase ammonium sulfate by about 1 ounce per plant, up to 6 to 8 ounces (3/4 to 1 cup) by the fifth year when plants are mature. At this time a balanced fertilizer (10-10-10) applied yearly in divided applications in spring and early summer can replace ammonium sulfate. Note that research links excessive phosphorus (the middle number in NPK) to iron chlorosis in blueberries, which need higher nitrogen, some potash, and little phosphorus. Rhododendron fertilizers may also be used.
For organic methods, instead of ammonium sulfate substitute materials with high nitrogen content. Organic fertilizers have a slower release time; apply them one to four weeks earlier than the schedule for soluble fertilizers. The only quick-acting organic fertilizer is blood meal (NPK 12-1.5-0.6) that lasts 6 to 8 weeks. Slower release nitrogen sources are acidic cottonseed meal (6-2.5-1.7), feather meal (13-0-0), fish meal (10-4-0), soybean meal (7-1.6-2-3) or alfalfa meal (3-1-2). Fertilizers can be used in three split applications from early spring until late June. For trace minerals use kelp meal lightly under mulch or mixed into mulch around bushes.
Blueberries respond well to spraying liquid seaweed/fish emulsions on plants. Apply at bud break, before harvest and just after harvest. Compost tea sprays also give plants a boost.
Blueberries must have adequate moisture, but plants can drown in waterlogged soil. Do not let plant roots dry out. Begin irrigation or sprinkling before soils dry out. Keep a consistent soil moisture level by applying 1 inch of water weekly during summer as needed. From blossom set until the end of harvest plants need water, especially as berries swell and ripen. From late July into August moisture is needed as plants begin fruit bud formation for the next year's crop.
The number one pest for blueberries is a bird. Birds will begin harvesting before berries turn color and won't stop until every berry is gone. They do not share! The only effective way to save a crop from birds is to net the bushes completely with bird netting. Brittle blueberry branches are easily broken and birds will reach through if bushes are draped with netting. The best solution is to make a frame higher than the tallest bush out of PVC pipes, place netting over the frame, and weight down bottom of nets with boards or metal pipe. Birds will find a way into the smallest opening and create havoc when they try to get out. Otherwise, blueberries are quite pest free. Bacterial canker can appear early in the year as irregular brownish-black areas on one-year-old wood. Use a copper spray in October and prune out any diseased wood. Mummy berry is caused by fungi overwintering in fruit mummies on the ground. Infected berries turn reddish-buff and become shriveled, hard and gray. Remove any berries and burn to break the cycle. Allow good air circulation around bushes. Consult your county extension service for more information on insects and diseases if you have a problem.
Blueberry varieties ripen at different times from mid-July until late August. Unlike most fruits, blueberries will hold on the bush for several days after ripening. Most people try to harvest berries too early. Berries turn from green to red to purple, and then blue with a waxy covering that protects berries from spoiling. Don't rub it off! Their full flavor peaks a few days after they turn blue and berries increase in size during this period. Ripe berries will come off the bush easily. Use a gentle rolling motion to pick into shallow containers. After the first picking berries in clusters will have more room to develop and ripen. Plan to pick several times in a season. Do not pick when berries are wet. Blueberries freeze well as dry fruit placed in freezer containers.
Blueberries first send up straight, unbranched green canes that turn woody as they age. The tips turn black and halt terminal growth. The next year these primary canes develop lateral branches, which will also die at the tips, and secondary branches will form. Lower buds develop when the tips stop growing and branching. New canes will continue to develop from the roots and repeat the pattern. If left unpruned, eventually the bush becomes twiggy with smaller berries. Fruit buds are formed at tips of branches on one-year-old wood and appear fatter than leaf buds. Tip pruning would destroy the fruit crop.
To discourage diseases, prune only when weather is dry. Begin a pruning program in the third year of growth. Keep one cane for each year of age plus two extra new canes. Cut back one of the canes if growth is less than one inch in diameter. Remove any diseased or winter damaged branches by cutting back to healthy wood. Remove any low branches less than one foot from the ground or any crossed or rubbing branches. Keep the center quite open for good air circulation and sun to produce sweeter fruit. Remove twiggy growth and any weak lateral shoots on older canes. The largest berries are produced on branches that are 1/5 inch in diameter with 1/10 inch diameter branches producing small berries. Prune out these tiny twigs.
By the fifth year the plant will have matured and canes will no longer be productive. Growth on these canes will have small, weak lateral branches and few berries. An overcrowded bush with old canes will not produce new replacement canes. Make it a practice to remove a five-year-old cane to the ground every year. Remove 5-year-old canes to the soil line or to a strong new lower side shoot. Removal of old canes encourages new canes to grow from the roots. Two or three new canes per year will be replacements for other aging canes. A mature plant should have 6 to 8 canes plus two new canes. Overbearing can be a problem at times with too many small berries set and little new cane growth. Remove 1/3 of flower buds and tip back some of the weakest young wood. These canes are about one inch in diameter near the base. Bushes can be pruned in September in mild winter areas, but pruning is usually done in winter while plants are dormant. Inspect plants while leaves are on. Leaves can tell you the quality of the stems. Keep big stems with big leaves and plan to remove those with small leaves. The key point in pruning is to maintain plant vigor. Yearly pruning will keep blueberry bushes in good production for many fruitful years.
Old, neglected blueberry bushes can be rejuvenated and brought back to production. Cut the entire bush to the ground. Fertilize as needed and mulch. The first year new canes will grow and the second year fruiting will begin. Blueberries produce the best crops on 2- to 4-year-old wood.
This article was presented as a handout for the talk, "History and Cultivation of Blueberries", given by Marie Townsend at the Home Orchard Society's 2005 All About Fruit Show.