Optimizing Planting Density to Increase the Sustainability of Blueberry Farms

During the first year of establishment, plant size appeared to be genotype-dependent, with ‘Optimus’ SHB developing larger plants than ‘Meadowlark’ SHB across all planting densities. Plant size in ‘Meadowlark’ SHB was unaffected by planting density. In contrast, ‘Optimus’ SHB plants in the high-density treatment were  approximately 25 % shorter than those growing in the standard (std), and developed approximately 30 % smaller canopies than those plants at low and std planting densities (Figure 1).  

Figure 1. Plant height, east-west (EW) and north-south (NS) canopy diameters of southern highbush blueberry across different plant density treatments. Means within the same genotype with no letter in common are significantly different (Fisher’s LSD; α = 0.05)

‘Optimus’ SHB plants exhibited higher VPLA than ‘Meadowlark’ SHB plants likely reflecting the canopy diameter differences observed in Figure 1. In ‘Meadowlark’ SHB, VPLA did not vary between treatments during the first year of establishment (Figure 2). In contrast, ‘Optimus’ SHB plants in the std- and low-density treatments had significantly higher VPLA in October 2024, although these differences disappeared later in the production season, and VPLA became similar across all treatments (Figure 2). ‘Optimus’ SHB is characterized by a vigorous, spreading canopy, while ‘Meadowlark’ SHB tends to develop a more upright and compact canopy. At higher planting densities, limited space constrained growth in ‘Optimus’ SHB, but ‘Meadowlark’ was largely unaffected, achieving VPLA values similar to ‘Optimus’ SHB (Figure 2). Physical space enables a plant to create and develop shapes by differentially controlling their growth (Prusinkiewicz and Barbier de Reuille 2010). This warrants further investigation as plants continue to develop and reach maturity.

Figure 2. Vertically projected leaf area of ‘Optimus’ and ‘Meadowlark’ southern highbush blueberry across different plant density treatments throughout the first year of establishment. Means within the same assessment date with no letter in common are significantly different (Fisher’s LSD; α = 0.05).

Planting density did not significantly affect yield during the first year of establishment. Yield differences were primarily attributed to cultivar performance, with ‘Optimus’ SHB producing higher yields than ‘Meadowlark’ SHB. Nevertheless, as plants continue to grow, changes in yield and in other important traits are expected especially as plants approach maturity (3-5 years old).

We measured WUE by looking at how much biomass the plants produced for every unit of water they received. Our assessment does not consider water inputs from precipitation. Instead we focused only on irrigation water. WUE was similar across all planting densities for each genotype. However, ‘Optimus’ plants exhibited greater WUE than ‘Meadowlark’ (Figure 3) in all treatments except the high-density treatment, consistent with the VPLA patterns described above. Previous studies suggest that smaller-canopy plants at higher densities develop more efficient light-intercepting structures, maximizing total photosynthesis and supporting higher yields and growth (Anthony et al. 2020). Whether this advantage persists as blueberry plants mature and dense canopies limit lower-light penetration remains to be investigated.

In both genotypes, most of the biomass went into cane growth rather than fruit or leaves (Figure 4).

Figure 3. Water use efficiency (g DW L^-1 irrigation water) across planting density treatments of southern highbush blueberry plants in Florida. Means with no letter in common are significantly different (Tukey HSD; α = 0.05). G = genotype; PS = planting density.

Figure 4. Dry weight of different plant organs of southern highbush blueberry plants during the first year of establishment. Means with no letter in common are significantly different (Tukey HSD; α = 0.05). G = genotype; T = plant organ.

‘Optimus’ SHB plants exhibited higher N, P and K use efficiency than ‘Meadowlark’ SHB at the standard planting density (Figure 6). Within the same genotype, ‘Optimus’ exhibited similar nutrient use efficiency across the distinct planting density treatments, although a slight decline was observed at the low-density treatment. On the other hand, ‘Meadowlark’ SHB plants exhibited highest nutrient use efficiency at high planting density than at standard- and low-density treatments (Figure 5).

Higher planting densities in other crops exhibited improved rhizosphere conditions (e.g., root activity, microbial abundance and diversity, and nutrient cycling), which can indirectly enhance nitrogen uptake (Wu et al. 2024). Although we did not measure soil physical or chemical properties in this study, the more uniform canopy coverage at higher densities may have contributed to improved soil conditions, including reduced water loss by evapotranspiration thereby enhancing nutrient uptake (Figure 5). Because each genotype develops differently, further research is needed to determine how long the benefits of a uniform canopy persist, when they begin to diminish, and how they influence other key traits such as yield and fruit quality. While WUE differences were primarily driven by genotype during the first year of establishment, future results may change as plants grow and compete more intensely for resources like water. Understanding these dynamics can guide planting strategies that support strong growth from establishment through maturity.

Figure 5. Nitrogen (left), phosphorus (middle) and potassium (right) use efficiency of southern highbush blueberry plants at different planting densities. Means with no letter in common are significantly different (Tukey HSD; α = 0.05). G = genotype, PS = planting density.