Highbush blueberries are long-lived, perennial, deciduous, woody shrubs that belong to the Ericaceae (heath) plant family. Other members of the Ericaceae family include cranberry, lingonberry, huckleberry, rhododendron, azalea, and heather.
Highbush blueberry plants usually require 6 to 8 years to reach full production and range from 5 to 8 feet high at maturity. The climate and soils in the PNW are conducive for good growth, development, and high yields. Depending on the region, average yields of mature blueberry plantings range from 7 to 10 tons per acre for early-season cultivars such as ‘Duke’, and even greater in many mid- and late-season cultivars such as ‘Liberty’, ‘Legacy’, ‘Elliott’, and ‘Aurora’. A good understanding of plant physiology is important for good management practices to sustain these high yields and achieve good quality fruit over time.
In this presentation, I will cover aspects of root and shoot growth, photosynthesis, transpiration, source to sink relationships, flower bud initiation, stages of dormancy, and plant growth. Specific aspects of plant growth that I will address include flowering and berry development, which are also important to better manage blueberry plants. This proceedings article is a companion to the Power Point presentation provided separately.
Blueberry plants have a very fine and shallow root system. Roots that are larger than 1 mm in diameter serve mainly to anchor the plants and transport water and nutrients to the shoots. The next type of root are dark brown and are long-lived (ranging from 150 μm, a little more than the thickness of a human hair, to 1 mm in diameter); these mainly transport water and nutrients. The finest roots (range from 40 to 75 μm in diameter) are white or light brown in color and primarily function to take up water and nutrients from the soil; these roots are short-lived, with a lifespan of only 115 to 120 days, and are sometimes referred to as “feeder” roots. The finest roots are often colonized by mycorrhizal fungi – these fungi help the blueberry plant take up nutrients. The smaller “feeder” roots have a short life-span, with half the plant’s roots dying within 100 days. These shed roots are replaced by new root growth. The roots are thus sensitive to plant and environmental factors. Irrigation, fertilization, mulch type, and raised beds vs. flat ground can all influence blueberry root system growth and development.
Blueberry plants produce new roots throughout most of the year. Root growth usually begins in early-April, when soil temperatures reach about 55 °F, and continues through early-November. We often find two peaks in root production in Oregon. The first peak occurs in late-spring or early-summer and is triggered by high water and nutrient demands for fruit production. The second peak occurs in mid- to late-summer, after harvest, as the plants begin to accumulate carbohydrate and nutrients in preparation for dormancy (Valenzuela et al., in progress).
In general, blueberry roots do not extend very far from the plant. In most soils, 50% to 60% of the roots are located in the top foot of soil and are within 8 to 12 inches from the crown. However, this rooting pattern can depend on management factors. For example, plants irrigated by drip have roots concentrated near the emitters, while those irrigated by sprinklers tend to produce a uniform layer of roots. Plants also produce more roots when grown on raised beds than on flat ground and produce deeper roots with weed mat than with sawdust mulch (Valenzuela et al., in progress). In sandy soils, roots will grow deeper than in clay or silt loam soils. In Florida, where a pine bark system is often used, blueberry roots will only grow in the upper pine bark layer, which has the preferred lower soil pH and a higher organic matter than the lower sandy soil.
Shoot types and growth
There are generally three types of shoots in blueberry: 1) suckers that develop from buds on roots; these are typically only found in rabbiteye blueberry cultivars; 2) whips that develop from latent buds (buds that have been dormant for at least one year) on older wood at the base of the crown or higher up on the bush from older wood; found in all blueberry types; and 3) lateral shoots – these develop from vegetative buds on one-year-old wood (last year’s growth). Whip growth occurs later in the season than lateral growth, typically starting in June in Oregon’s Willamette Valley. The number of whips per plant is very affected by pruning severity and light exposure to the base of the plant. Some cultivars do not produce whips from the base of the plant, instead producing most whips from higher up on the bush (e.g. ‘Duke’, ‘Berkeley’).
The growth of all types of blueberry shoots occurs in “flushes”. At the end of each growth flush, the bud at the tip of the short aborts, dies, and turns black. This small black tip is referred to as the “black tip” stage. Growth will often continue from an adjacent bud. Most highbush blueberry cultivars have one to three flushes of growth per growing season. Whips often have more growth flushes than lateral shoots. The number of flushes varies with cultivar, length of growing season, vigor (pruning severity or crop load), and management (particularly fertilization with nitrogen). Growth must slow in late summer for flower bud development to occur (see below). Late flushes of growth are also more sensitive to frost damage in autumn.
Plant Growth and Development
Growth of blueberry is first visible in the spring with the onset of "bud swell" on one-year-old wood (last year’s growth). This one-year-old wood has vegetative buds (small, scale-like buds that will only produce a shoot with leaves) and may contain flower buds (larger and wider buds that only contain a flower cluster). Flower buds, if present, are located at the tip portion of the one-year-old wood.
Shoot growth occurs rapidly in the spring and begins to slow in midsummer. Floral bud initiation (initial development of flower buds) begins to occur late summer to early fall as days get shorter and night temperatures cool. The number of flower buds per shoot is related to the number of days of good weather for flower bud initiation. For example, ‘Duke’ will produce more flower buds per shoot when grown in Oregon’s Willamette Valley than in Michigan, which has a shorter developmental period in autumn. Flower bud initiation starts at the tip of a shoot and progresses downward – shoot growth must thus slow or stop before initiation can occur. Factors that promote late shoot growth thus reduce fruit bud set. Other factors that affect flower bud initiation include light intensity (less in shade), plant carbohydrate status (see below), and cultivar. Typical fruit bud set (the proportion of buds on a lateral that become fruit buds) in the Willamette Valley, Oregon ranges from 22% to 55% (Strik and Vance, in progress).
Once flower buds have initiated, flower bud differentiation (development of flowers and flower parts within the buds) occurs. There is a strong cultivar effect on the number of flowers that develop within each bud. For example, in an 8-year study, ‘Duke’, ‘Draper’, ‘Bluegold’ and ‘Legacy’ had about 7 flowers per bud; ‘Reka’, ‘Bluejay’, ‘Liberty’, and ‘Aurora’ had 8 flowers/bud, and ‘Bluecrop’ and ‘Ozarkblue’ had 9 to 10 flowers/bud (Strik and Vance, in progress).
When temperatures become too cool for plant development, flower bud differentiation will cease – it typically continues again in later winter/spring prior to bloom. Plants then start acclimation or entering dormancy. Acclimation is the gradual physiological process by which a blueberry plant adjusts to colder winter temperatures. This acclimation process results in increased winter hardiness and is initiated by decreasing temperatures and daylight hours. The degree to which a blueberry plant “hardens off” in the fall depends on multiple factors, including the length of the growing season, photoperiod, alternating day and night time temperatures, nutrition, pruning, and fluctuating temperatures. The length of the acclimation phase depends on the climate and the cultivar. Once plants are dormant, they require a period of cold temperature (chilling requirement) before they will grow normally. Northern highbush cultivars have a chilling requirement of 800 to 1500 hours (between 32 and 45 °F), whereas southern highbush and rabbiteye blueberry cultivars have a chilling requirement of 200 to 600 hours and 300 to 600 hours, respectively. While rabbiteye blueberry bloom relatively late, southern highbush will bloom very early in late-winter or spring, after chilling is satisfied, making these poorly adapted to the Pacific Northwest. After chilling is satisfied, the plants enter a deacclimation phase. After this phase is over, bud break occurs once temperatures are warm enough.
Cold hardiness increases from the beginning to the end of the acclimation phase in autumn. Once plants are dormant, they are at their maximum cold hardiness. During the deacclimation phase in late winter to spring, cold hardiness decreases. Blueberry cold hardiness varies tremendously across species and cultivars. Fully dormant northern highbush cultivars are considered cold hardy to at least -13 °F when fully dormant. However, ‘Legacy’, which is part southern highbush, is considerably less cold hardy, as are rabbiteye blueberry cultivars. The overall level of cold hardiness varies among plant parts. The wood is more cold hardy than the buds. Also, flower buds are less cold hardy than vegetative buds. Open flowers are damaged when temperature is about 28 °F. The level of damage depends on the stage of plant development, how rapidly temperatures change during a cold event, and how cold it gets.
Cultural practices that promote late fall growth can disrupt acclimation and inhibit the development of maximum cold hardiness. For example, excessive or late fertilization with nitrogen encourages late-season growth that is susceptible to early fall frosts. In warm regions, plants may not enter dormancy (e.g. “evergreening” of southern highbush in sub-tropical climates).
Flowering and berry development
Flowering occurs in spring when floral buds swell and open (“bud break”). Floral buds near the tip of one-year-old shoots open first, followed by the buds below. Timing and duration of bloom vary by cultivar and prevailing weather conditions. The flowers of highbush blueberry are urn- shaped and consist of the: 1) calyx, which ends up at the tip of a berry; 2) the corolla, or fused petals; 3) about 10 stamens that each have a pollen-containing anther at their tip; and 5) a pistil, with an ovary at the base containing many ovules (which become seeds if they get fertilized by pollen). Flowers are receptive to pollination and seed set for 3 to 5 days after flower opening. Some cultivars will not set many seeds unless pollen comes from a different cultivar (low “self- fertility”). Rabbiteye and southern highbush cultivars are not very self-fertile, needing a pollinator for good fruit production. Flowers need a large number of visits from bees (the main pollinator) or other pollinators for good fruit and seed set. Fruit set (the proportion of flowers that become berries) averaged 93% in an 8-year study in Oregon’s Willamette Valley (Strik and Vance, in progress). Fruit set may be lower in regions that get a lot of rain or cold weather during bloom, which reduces bee activity.
After pollination and fruit set, the berry goes through three phases of growth: 1) cell division, where the berry increases in size but is still green; 2) embryo development, where the berry does not increase in size much; and 3) cell expansion, where each cell increases in size. The development of pink and then blue color starts at the end of the second phase of berry growth. Sugars increase and acids decrease during phase three. Berry weight is very dependent on cultivar, crop load (severe pruning will increase berry size compared to light pruning), stage of development (berries increase in size after they first turn fully blue), and the number of seeds per berry (in most cultivars). Berry firmness is mainly affected by cultivar, but is also impacted by stage of fruit harvest (ripeness of fruit), cultural practices, and weather.
Photosynthesis, transpiration, and source-sink relationships
Photosynthesis is the process by which plants use the energy of sunlight, carbon dioxide, and water to make carbohydrates. Inorganic salts, chlorophyll (the green pigment in plants), and other catalysts are important in this process. Many of these organic salts and catalysts are taken up as nutrients from the soil solution. In blueberry, photosynthesis may be limited by light, temperature, and water. For maximum production of carbohydrates, none of these factors are limited.
Water initially moves into the root by osmosis, because the solute content (dissolved chemical components) of the root cells is higher than that of the surrounding environment. This creates a root pressure that extends into the xylem cells (or water pipes of the plant). Water moves out of the plant as a vapor through the somata (pores) on leaves – this is called transpiration. This loss of water creates a pressure deficit and causes water movement through the plant. Transpiration is affected by light and water, which affect stomatal opening, as well as relative humidity and temperature, which affect the rate of water evaporation. Wind also influences transpiration, with higher wind velocities leading to increased transpiration. Transpiration has an important cooling effect for leaves and the plant, but can lead to excess water loss if the water is not replaced by rain or irrigation.
In the plant, there is a flow of carbohydrates from the “source” of production (leaves) or a storage source (e.g. roots, crown, and older canes) to a depository known as the “sink” (e.g. fruit, shoot tips, and root tips). The supply of carbohydrates by the “source” must be greater than the demand by the “sink” or growth of the “sink” will be limited. Sink strengths also vary in importance. Fruit are a stronger “sink” than flowers, which are stronger than shoot or root tips, and shoot and root tips are a stronger “sink” than the storage organs. The plant will allocate limited carbohydrates or resources to the sinks in this order of priority. Thus, if a plant is not pruned well, and there is too much fruit (the strongest “sink”) on it, then growth of the shoots and roots will suffer, and the plant will not store as many carbohydrates and nutrients as it would otherwise if it were “balance” pruned. Therefore, careful pruning is very important for balancing future vegetative plant growth with production of high quality fruit.