Strawberries 201

Producing hydroponic strawberries is all about the fruit quality.

The value proposition of hydroponic strawberries justifying their higher prices compared to field-grown fruit is due to the superior fruit quality. Strawberries are unique in the controlled environment food crop world, as they stand apart from leafy crops (i.e., greens and herbs) and fruiting vine crops (i.e., tomatoes and cucumbers) in their production techniques. The fundamentals of strawberry production were covered in an earlier article, while this article is going to present opportunities to further improve strawberry productivity and quality. (Editor’s note: Read Strawberries 101 from the April 2018 issue at bit.ly/strawberries-101)

Producing high-quality strawberries with a high Brix or sugar concentration begins with the plant. Carbohydrates stored in the crown and those produced from photosynthesis are assimilated into the achenes or fruits as they develop. There are two approaches to managing strawberry plants that can help promote high-quality fruits: 1) establishing strong and healthy plants; and 2) maintaining a balance between vegetative growth — the canopy — and fruit development.

When strawberry plants are planted, become well-established and leaf unfolding and expansion are promoted, more and higher-quality fruits can develop. Several factors can help promote establishment of newly planted transplants in production systems. First, aim to provide environmental conditions that promote growth. Although strawberry plants are cold-tolerant, growing too cold will inhibit root growth and leaf unfolding and expansion. Similarly, avoid establishing plants under low-light conditions. Also, manage plant culture to promote establishment by avoiding overwatering or high electrical conductivity (EC), both of which can inhibit root development.

Strawberries, like in fruiting vine crops, are grown when vegetative growth (leaves) are balanced with generative or reproductive growth (flowers and fruits). This concept is centered around source-sink relationships, where leaves are the source of photosynthates (carbohydrates) and fruits are the sink for them. If greenhouse conditions and cultural practices are such that growth and development of the plants themselves is not robust, do not allow all flowers to develop into fruits. A large number of fruits developing on a plant that can’t support them will result in fruit that doesn’t develop with the quality needed to command the premium price for greenhouse strawberries. Alternatively, if crown and canopy development is robust, maximum fruit production can be promoted.

A dynamic approach to managing mineral nutrient solutions is another way to improve fruit quality. First, start by altering the ratio of mineral nutrients during production. When strawberries are first planted into systems for establishment, maintain a nitrogen (N) to potassium (K) ratio of 1:1. This balance of N to K will promote good vegetative growth, i.e., leaf development. Once the crop is starting the fruiting phase of production, increase the K relative to N in the solution to an N:K ratio closer to 1:2. This is like the shift in N and K ratios seen in fruiting vine crops. Although strawberries are not a high-wire vining crop, they are a fruiting crop, and fruits have a higher demand for K than leaves.

In addition to altering the ratio of mineral nutrients, the strength of the nutrient solution or EC can also be altered to promote fruiting. First, once strawberry plants are established in their production systems and well-rooted, the EC of the nutrient solution should increase (somewhere between ~0.5 and 1.0 mS/cm). Some research has shown increasing EC even more can enhance the flavor of strawberries, but yield is reduced. This concept is similar to elevating the EC to enhance flavor of tomatoes, which also show reduced yields with spiked EC.

Strawberry production continues to increase with the interest in high-quality and high-value food crops grown in controlled environments. Once the fundamental aspects of managing the growing environment and cultural practices are under control, try using the techniques outlined in this article to further improve strawberry production and fruit quality.

Christopher (ccurrey@iastate.edu) is an associate professor of horticulture in the Department of Horticulture at Iowa State University.