How blueberry plants grow and develop

Blueberries are perennial berry-producing shrubs, and their cultivation is becoming increasingly popular in Georgia. Georgian growers face various challenges, as many biotic and abiotic factors affect plant growth, development and fruit formation.

We are sharing this article from Oregon State University to help you better understand the crop.

Highbush blueberry cultivars generally reach full production in 6-8 years. The climate and soils of the Pacific Northwest are well suited to blueberry production. Depending on the region, the early cultivar Duke yields an average of 17-25 tonnes per hectare. Mid- and late-season cultivars such as Liberty, Legacy, Elliott and Aurora produce similar yields. Understanding plant physiology is essential for adopting good growing practices and consistently achieving high yields and excellent fruit quality.

Effective plantation management requires an understanding of the root system, shoot growth, photosynthesis, transpiration, flowering and dormancy.

Roots

Blueberries have a shallow, fibrous root system. Roots more than 1 mm in diameter anchor the plant and transport water and dissolved nutrients to its shoots.

The next type consists of longer-lived, dark brown roots, ranging from 150 microns to 1 mm in diameter, slightly thicker than a human hair. Their main function is to absorb water and dissolved nutrients.

The finest roots, 40-75 microns in diameter, are white or light brown. They are relatively short-lived, lasting 115-120 days, and are the most active feeding roots. They are often colonised by mycorrhizal fungi, which help the plant absorb soil nutrients in exchange for small amounts of carbohydrates. Because these roots have a short lifespan, the plant continually replaces them with new fine roots.

Blueberry plants produce new roots throughout the year. Root growth generally begins in early April when soil temperatures reach approximately 10-12 degrees.

Root growth has two peaks. The first occurs in late spring or early summer, when demand for water and nutrients is high. The second follows harvest, as plants begin storing carbohydrates and nutrients in preparation for winter.

Blueberry roots generally do not extend far from the plant. In most soils, 50%-60% of the roots are close to the surface and within a 30 cm radius of the bush centre. Root development depends on many factors. Under drip irrigation, roots concentrate near the emitters; under sprinkler irrigation, they are distributed more evenly near the soil surface. Plants on raised beds develop larger root systems than those on flat ground, while roots grow deeper under mulch made from weeds than under sawdust mulch.

Roots grow deeper in sandy soils than in clay or loam. Blueberry roots perform better in soils with a low pH.

Keep in mind that blueberry roots are highly sensitive to environmental conditions. Irrigation, fertigation, mulch type and raised beds can all have a major effect on root growth and development.

Shoot types and growth

Blueberries generally have three types of shoots:

1. Suckers develop from adventitious buds on roots. This type is mainly associated with rabbiteye cultivars.
2. Central-leader shoots develop from latent buds that have passed through at least one dormant period. They occur in all blueberry cultivars.
3. Lateral shoots develop from vegetative buds on the previous season’s growth.

Central-leader shoots begin growing later than lateral shoots, generally in June in Oregon. Their development depends on pruning intensity and light distribution within the plant. Some cultivars, such as Duke and Berkeley, do not produce leaders from the base, instead developing most of them in the upper part of the bush.

The amount of growth produced at an unsuitable time varies with the cultivar’s growing-season length, vigour, crop load and management, especially the timing of nitrogen fertilisation. Growth should slow in late summer to allow flower buds to develop well. Late shoots are more susceptible to frost damage in late autumn.

Plant growth and development

The number of fruiting buds on a shoot is related to the number of days with favourable weather. Open flowers are damaged when temperatures fall to -3 degrees. The extent of damage depends on the plant’s developmental stage and how quickly temperatures change.

The first visible spring growth begins with budbreak on the previous season’s shoots. A branch has vegetative buds: small, scale-like buds that produce shoots with leaves only. It may also carry flower buds, which are larger, broader and produce only reproductive growth. Fruiting buds are located towards the tips of annual shoots.

Shoots grow rapidly in spring and begin to slow in midsummer. Flower buds develop in late summer and early autumn as days shorten and night temperatures fall.

For example, Duke produces more fruiting buds per shoot in Oregon than in Michigan, where the autumn development period is shorter.

Flower-bud formation begins at the tip and progresses downwards. Shoot growth must slow or stop before this process begins. Factors that encourage late shoot growth reduce fruiting-bud formation.

Flower-bud initiation is followed by differentiation: the development of flowers and flower parts within the buds. The number of flowers per bud depends on the cultivar. An 8-year study found that Duke, Draper, Bluegold and Legacy had 7 flowers per bud; Reka, Bluejay, Liberty and Aurora had eight; and Bluecrop and Ozarkblue had 9-10.

Flower-bud differentiation stops when temperatures become too low for plant development. It generally resumes in late winter or spring before flowering. Plants then acclimatise or enter dormancy. Acclimatisation is the gradual physiological process by which blueberry plants adapt to cold winter temperatures.

Falling temperatures and shorter days trigger acclimatisation. The degree of autumn hardening depends on factors including growing-season length, photoperiod, day–night temperature differences, nutrition, pruning and temperature fluctuations. The duration of acclimatisation varies with climate and cultivar.

Once dormant, plants require a period of cold before they can resume normal growth. Northern highbush cultivars need 800-1500 chilling hours at 0-7 degrees, while southern rabbiteye cultivars require 200-600 hours. After the chilling requirement is met, plants enter deacclimatisation. Budbreak follows once this phase is complete and temperatures are sufficiently warm.

Cold hardiness increases throughout autumn acclimatisation and decreases during deacclimatisation from late winter into spring.

Cold hardiness varies greatly among blueberry species and cultivars. Northern highbush cultivars tolerate temperatures of -25 degrees, while Legacy, a southern highbush cultivar, is less frost-hardy. Frost response also differs between plant parts. The canopy is colder than the buds, while fruiting buds are less cold than vegetative buds.

Practices that encourage late-autumn growth can affect cold hardiness. Excessive or late nitrogen fertilisation stimulates late-season growth that is vulnerable to early autumn frosts. In warm regions, plants may not enter dormancy.

Flowering and fruit development

Flowering occurs in spring as flower buds open, beginning near shoot tips and progressing to lower buds. Its timing and duration depend on the cultivar and weather. Blueberry flowers are urn-shaped and consist of the following parts:

The calyx, which remains at the tip of the berry; a corolla of fused petals; and approximately 10 stamens, each with a pollen-containing anther at its tip.

A pistil with an ovary at its base containing numerous ovules, which become seeds if fertilised by pollen.

Blueberry flowers remain receptive to pollination for 3-5 days after opening. Rabbiteye and southern highbush cultivars need pollination partners to produce high-quality fruit. An abundant population of bees or other pollinating insects is also required.

Long-term observations in Oregon show average fruit set of 93%. The percentage may be lower in regions with frequent rain and low temperatures during flowering, as these conditions reduce bee activity.

After pollination and fruit set, blueberries pass through three growth phases:

1. Cell division: the berry increases in size but remains green. 2. Embryo development: the berry reaches its maximum size and stops growing. 3. Cell enlargement: individual cells increase in size.

Pink and then blue colour develops towards the end of the second phase. In the third phase, sugar content increases and acidity decreases. Berry weight depends on the cultivar, crop load, developmental stage and number of seeds. Heavy pruning increases berry size, and berries continue to enlarge after turning fully blue. Firmness depends on the cultivar, ripeness, growing practices and weather.

Some cultivars’ flowers are not fertilised unless pollen is brought from the pistil of another cultivar.

Photosynthesis and transpiration

Photosynthesis is the process by which plants use sunlight, carbon dioxide and water to synthesise carbohydrates. Inorganic salts, chlorophyll, the green plant pigment, and other catalysts are important to this process. Most of these organic salts and catalysts are absorbed from the soil. In blueberries, photosynthesis may be limited by light, temperature and water availability. None of these factors should be limiting if carbohydrate production is to be maximised.

Water initially enters the roots by osmosis because the concentration of chemical compounds inside the root is higher than in the surrounding environment. This creates root pressure, which extends into the xylem cells, the plant’s water-conducting channels. Water evaporates from the plant through stomata, or pores, on the leaves. This process is called transpiration.

This water loss creates a pressure deficit and drives water movement through the plant. Light and water availability influence pore opening, while relative humidity and temperature affect the rate of evaporation.

Wind also affects transpiration, with strong winds increasing it. Carbohydrates move from their production source, the leaves, or from storage tissues in the roots, canopy and perennial branches. Supply must exceed demand; otherwise, growth is limited.

Incorrect pruning can lead to overcropping, reducing vegetative and root growth and leaving the plant with fewer stored carbohydrates and nutrients than under appropriate pruning. Choosing the right pruning method is important for balancing future vegetative growth and producing high-quality fruit.

Agronomist’s recommendations:

Use agronomic practices suited to the conditions of each plantation so that the chosen cultivar can perform at its best. IOTO Agro offers Spanish fertilisers and growth regulators that meet Global GAP standards and have been used successfully in Georgia for years. We believe that high-quality products meeting European standards can deliver the best results in our plantations.

Article prepared by:

Agronomists: Tazo Doborjginidze and Lasha Tsigriashvili.

Source: https://extension.oregonstate.edu/crop-production/berries/how-blueberry-plants-develop-grow