Cultivar Selection Is Key to Radish Production
July 5, 2023 
A 72-cell tray of spring radishes with shoots removed to show the variation in germination, radish size or grade, and damage. Photos: Christopher J. Currey
Note: This article, part one of a four-part series on hydroponic radish production, written by the team of Christopher J. Currey, Robert Muniz II, Ryan Niepegan, and Peter Konjoian. Currey is an associate professor, Muniz II is a former undergraduate research assistant, and Niepegan is a graduate research assistant, all in the Department of Horticulture at Iowa State University. Konjoian is a horticultural consultant with Konjoian’s Horticulture Education Services, Inc.
Radishes are a popular vegetable, grown for their enlarged fleshy taproot. Several different types of radishes are grown commercially, including spring, summer, and winter radishes. However, the most popular type of radish in the U.S. is the spring or table radish. Spring radishes are commonly produced outdoors in fields. But there are opportunities in the greenhouse, too.
Radishes are a promising crop for controlled environments. We found that producing radishes in plug trays filled with regular soilless substrate and grown using subirrigation is an effective and efficient way to produce a high-quality radish crop inside a greenhouse. Growing radishes hydroponically in the greenhouse provides a few advantages over field production. Quality can improve through regular irrigation, reducing cracking damage that would render a radish unmarketable. Additionally, minimizing stress by managing light, air temperature, water, and fertilizer can help radishes maintain a balanced flavor. Finally, the planting density in cell trays can be much greater compared to field spacing.
Unlike popular hydroponic crops such as lettuce, tomato, cucumber, or pepper, radishes do not have cultivars bred specifically for growing in controlled environments. As a result, radish cultivars developed for field production must be used for hydroponic production in controlled environments — but which will do best? Our objective was to quantify the productivity of a broad selection of commonly available spring radish cultivars grown in soilless production systems in greenhouses.
Materials and Methods
Twenty-seven radish cultivars were obtained from a variety of commercial vegetable seed suppliers, including Johnny’s Selected Seeds, NESeed, Rispens, Rupp, Seedway, and Siegers. Seeds were sown in 72-cell plug trays filled with a commercial soilless substrate and covered with a light covering of coarse vermiculite. After seeding, trays were moved into a glass-glazed greenhouse with a constant air temperature setpoint of 68°F and a target daily light integral of 12 mol∙m–2∙d–1.
Seeded trays were placed into one of two identical flood tables, with interior dimension measuring 36 inches W × 72 inches L × 7.4 inches H. Trays were initially hand-irrigated to saturation with clear water immediately after planting. Flood tables were flooded every other morning for the first two weeks, every morning for the third week, and in the morning and afternoon in the fourth week. Each flood tray had a 40-gallon reservoir filled with a solution consisting of tap water amended with 15-5-15 Cal-Mag (Peters Excel; ICL Specialty Fertilizers, St. Louis, MO) providing 150 ppm nitrogen.
Four weeks after seeds were sown, the research team collected data. Washed radishes were graded by hypocotyl diameter according to USDA grading standards. Additionally, radishes without swollen hypocotyls or those that were unmarketable due to deformity, cracks, or other factors were recorded as per USDA standards. The leaves were excised at the top of the radish and the fresh weight was recorded for each of the USDA grade categories.
This is an example of a mature spring radish grown in a 72-cell tray after 28 days and ready for harvest.
The Good, the Bad, and the Tasty
When it comes to germination, there were very few differences among the cultivars in this study. Around three-fourths of the cultivars in the study had germination rates of around 90% or better. ‘White Icicle’ and ‘German Giant’ were two radishes with notably lower germination rates (less than 80%).
Unlike containerized greenhouse plants, which are sold on a per-unit basis, food crops are most often sold by weight. Radishes may be sold by count, but are most commonly sold by weight. ‘Red Castle’ was the most consistent top-producer with respect to total marketable weight, with other notable cultivars including ‘Melito’ and ‘Red Crown’. In addition to total marketable weight, the number of marketable-sized radishes is also an important aspect of cultivar performance. ‘Crunchy King’, ‘Red Crown’, and ‘Red Castle’ were some of the most productive, with respect to marketable radish number.
Defects related to the growth of the radish, such as cracking or splitting, and deformities were observed. ‘Cheriette’, ‘Cherry Belle’, ‘German Giant’, and ‘Purple Plum’ were the cultivars with the highest proportion of unmarketable roots. Most of the damage wasn’t due to deformity, but to cracking. While this can be caused by inconsistent moisture (as seen in the field) we had consistent substrate moisture throughout the study.
We did not evaluate the tops, or leaf growth, for the cultivars in the study. While radishes are not grown for their leaves, selling radishes bunched by shoots — as opposed to already processed with tops removed — may serve as a marketing strategy to enhance the value-added nature of radishes as greenhouse-grown food crop. Although we didn’t collect data on foliage, we did not observe any cultivar with insufficient leaf production that would preclude bunching.
We hope the results we present here are a good start towards finding what radish may work the best for your greenhouse. However, the importance of doing some trialing on your own cannot be overstated. While you would likely see some similar trends as we did for several of the cultivars, the environmental conditions and cultural practices unique to your facility may yield differences. Successful cultivar selection is the first step in maximizing productivity and profitability for any controlled-environment food crop producer.
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