Soil-less Substrates for Greenhouse Strawberry Production

Austin M. Wrenn^1,2, Brian E. Jackson¹ and Mark Hoffmann^1*
1North Carolina State University, Department of Horticultural Science, 2721 Founders Drive, Raleigh, NC, 27695.
²Grower and Owner, Wrenn Farms, Zebulon, NC
Corresponding author: mark.hoffmann@ncsu.edu

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Soil-less substrates in pots or bags can be replaced year to year, eliminating the need for crop rotation and fumigation. This system is often used in conjunction with greenhouse or tunnel systems for environmental protection, and the crop can be placed at any height for more ergonomic and effective labor usage. Additionally, most of the structural and irrigation components of these systems can be reused for multiple years (10-15+), reducing the long-term cost and environmental impacts.

Two of the most used soil-less substrate components worldwide for strawberry production are coco fiber and peat-based mixes. Substrates such as bark, wood fibers, Canadian peat moss, and perlite are readily available and presently in common use in other industries, such as the ornamental nursery industry. Little information is available to assist strawberry growers in making appropriate substrate selections for their operation. We evaluated the performance of greenhouse grown strawberries (Fragaria x ananassa cv. Albion) in six substrate blends sourced from different soil-less material with the goal of assessing strawberry production in local source substrate material in comparison to the grower standards (coco coir and European block peat) for the Southeastern US.

Soil-less substrate mixes that were investigated in this study

Performance of the strawberry ‘Albion’ was investigated in six custom mixed soil-less substrates for two growing scenarios: A Spring planting (‘Experiment 1’) and a Fall planting (‘Experiment 2’). Following soil-less substrates were used:

1. (CF) 100% Coco Coir (=grower standard)
2. (PP) 50% Canadian Peat / 50% Perlite
3. (PC)  50% Canadian Peat / 50% Coco coir;
4. (PW)  50% Canadian Peat / 50% Wood Fiber (locally source pine);
5. (PB)  50% Canadian Peat / 50% Bark (locally source pine);
6. (EP) European Peat mix (BVB)

Premier professional grate Canadian peat moss was used for the 50/50 mixes. Coco Coir was washed and buffered. Perlite was horticultural grade perlite. Substrate raw materials were measured out by cubic feet volume and mixed together manually to create the 50/50 blends. Pulverized dolomitic limestone was added to PB, PW, PC and PP when mixing to bring substrate pH up to ~5.6 before starting the trial. Lime was incorporated into the substrates, uniformly mixed, left to sit overnight, and mixed again before being used. Sixteen 1.64 feet (=0.5 meters) long pots were filled with each substrate and checked by weight to make sure all were equivalent. Containers were filled level to the top of the container without compressing the substrate (Figure 1).

Planting material and planting dates

Strawberry (Fragaria x ananassa cv. Albion) mother plants or tips were received from Norton Creek Farms, Waynesville, NC. Mother plants were rooted in an indoor nursery at the farm. Daughter plants were then harvested when needed and rooted directly in 21 cell trays (~240 cc cell volume = tray plant) under a separate misting greenhouse at the farm. For Experiment 1, rooted tray plants were planted on 3/14/21. Plants were removed on 6/23/21 with a growing duration of 102 days. For Experiment 2, tray plants were planted on 9/27/21 and plants were removed on 6/12/22 for a growing duration of 259 days (Figure 2).

Greenhouse and Experimental Design

Each experiment contained the same substrate treatments (see list above). Each treatment was replicated four times in space in a randomized complete block design. Each experiment had a total of 24 experimental units, with 16 plants per unit. Each unit consisted of four half-meter containers (Bato Plastics), filled with the designated soil-less substrate (Figure 2) and four plants per container.

The trial was conducted inside of a commercial mid-tech multibay plastic covered greenhouse facility in Zebulon, NC.  Each greenhouse bay measured 100 feet (30.5 meters) long by 21 feet (6.4 meters) wide. The gutter height of the structure was 8 feet (2.45 meters) (Figure 3).

Gutters

Gutters were developed and custom built by the lead author. This system included an elevated growing platform for more ergonomic crop care and harvest activities. Sixteen-inch-wide sections were cut from expanded metal fencing panels and bent into U shape Gutters.  Rebar brackets held the gutters on top of two ~4’ tall posts which had been driven into the ground. To allow for the capture of drainage water from each individual unit, rows were split into 8-foot sections and installed on a ~2% slope. Twenty-inch-wide strips of white plastic were cut out of white greenhouse film (6 mm thickness, AT Films) and fastened over top of the gutter with the edges overlapping slightly. A modified stapler was then used to staple the plastic around the gutter frame to secure it into place (Figure 3).

Irrigation and fertility

Drainage leachate and fertigate output solution was collected using a funnel and enclosed bucket directly under the low end of the gutter. Buckets were sealed to prevent evaporation of the drainage water and they were cleaned frequently throughout the experiment to reduce the potentials for algae growth.

Plants were irrigated using an automated irrigation controller (Orbit Bhyve). Overall irrigation times ranged from 2 minutes 2 times per day in the coldest period to 4 minutes 12 times per day towards the end of the experiment on hot days. Water soluble fertilizers were mixed into an A/B stock solution and injected at each irrigation event using fertilizer injectors (Dosatron). The program was adjusted slightly as needed based on plant appearance and tissue testing results. Approximate targeted fertility program for the fertigation output are shown in Table 1.

Data Collection

Several environmental traits were recorded over the course of the experiments. Initial PAR (Photosynthetic Active Radiation) readings were collected to assess possible shading variation between blocks as well as between units. Temperature and relative humidity were recorded throughout the trials. Drainage water and irrigation feed solution analysis was conducted weekly for electroconductivity (EC), pH and volume of solution. Data were collected on the irrigation water supplied to the crop as well as the drainage from the irrigation solution that leached out of the bottom of the growing containers. The percentage of irrigation solution that exited the container as drainage was also collected (DP).

Following plant yield characteristics were recorded weekly: marketable yield in grams, non-marketable fruit yield in grams, marketable fruit number, non-marketable fruit number, and average marketable fruit weight in grams. Total soluble solids of ripe fruit (°Brix) and number of runners were collected every other week. Marketable fruit quality was determined based on what would be acceptable for direct consumer sales of fresh fruit. Marketable fruits were 5g or above in weight, not severely deformed or with any disease or pest damage. A white ring around the top of the fruit, often called “white capping” was acceptable as it is a common characteristic of fruits grown in NC greenhouse production.

Results

Experiment 1 (Spring Planting, 102 days, 2021)

No significant differences in marketable fruit number, marketable fruit weight, TTS, Number of runners or average berry size could be found between treatments (data not shown). Significant differences were seen in average drainage volume per week or drainage percentage between the different substrates.  Differences between treatments were observed in EC and pH readings. PW had a significantly lower average drain EC reading (0.815 dS/m) compared to the other treatments. EP had a significantly higher average drainage pH (7.46) and PC had a significantly lower pH reading (5.48.).

Experiment 2 (Fall planting, 259 days, 2021-2022)

Cumulative marketable yields for EP averaged 974 grams per plant (2.15 lbs. per plant.) PP had the lowest yields at 810 grams per plant (1.79 lbs.) For marketable number per plant, PW & EP had the highest at 40 berries per plant each and PP had the lowest at 34 berries. EP and PW also had the highest cull weights (122.2 & 120.25) as well as the highest cull numbers (10.5 & 10.15). PB had the lowest cull weight (94.3) and cull number (6.98.) (Table 2).

No significant differences were found between TTS, average marketable berry size, runner number, drain volume, and drain percentage. PC had the highest EC in drain, and CF had the lowest with 1.19 units. PW had the highest drainage pH at 7.28 and PC had the lowest at 6.5 (Table 2).

Average marketable berry size was similar for all tested substrates with size averaging 20.9 to 23.9 grams (.046 lbs. to 0.053 lbs.).

Conclusion

The results of this study suggest that any of the trialed substrate blends can be used to produce a single season berry crop inside a med-tech greenhouse in eastern North Carolina. For short season cropping, soil-less substrate choice does not have an impact on yield performance of ‘Albion’. In the more common fall planting season, strawberries are grown for 200+ days. Our data suggest that ‘Albion’ grown in European Peat significantly outperformed ‘Albion’ grown in a Canadian Peat / Bark mix and a Canadian Peat / Perlite mix. However, ‘Albion’ grown in Canadian Peat / Wood Fiber mixes performed as well as those grown in European Peat, Canadian Peat / Coco Coir or in Coco Coir itself.  Our results suggest that Canadian Peat when mixed 50:50 with Wood Fiber might be a valuable and more local alternative to Coco Coir and European Peat.

Funding

North Carolina Department of Agriculture