Turning Up the Heat on Greenhouse Radishes

Editor’s note: This is the final installment of a four-part series on hydroponic radish production. Read parts one, two, and three here.

Spring radishes are well-known as a cold-tolerant crop, and they are often produced when outdoor temperatures are cool. However, we have the ability to control air temperature in controlled environments. This gives producers the ability to control and manipulate temperature not only to meet crop requirements, but also production schedules and goals. Temperature has a very strong influence on the rate of crop growth and development, though the effects of temperature on crops are often species-specific. Plants vary in their response to air temperature, most notably in their tolerance to cool and warm temperatures, as well as the degree of response to changes in air temperature. However, the response of radishes to air temperature has not been characterized well.

This is the fourth and final article in our four-part series on hydroponic radish production in controlled environments. So far, we have discussed the importance of cultivar selection, as well as how to manage mineral nutrition and light. This last article will highlight the effects of air temperature on radish growth and development and how to manage temperature for commercial production.

Materials and Methods

Seeds of ‘Crunchy King’ and ‘Red Castle’ were sown in 72-cell plug trays filled with a commercial soilless substrate. Trays were then cut in half to create two 36-cell sections to accommodate the environmental chambers used in the experiment. Seeds were covered with a light (~1/8 inch) amount of coarse vermiculite and irrigated to saturation with tap water, after which trays were transferred to a commercial portable germination chamber maintained at a target 70°F with 100% relative humidity and no light.

Following germination, trays were placed in one of six climate-controlled environmental growth chambers, each providing a different target air temperature: 46, 55, 65, 73, 82, or 91°F. Light was provided in each environmental chamber with a combination of fluorescent and incandescent bulbs, providing a target intensity of 200 µmol·m-2·s-1 for 16 hours, followed by an 8-hour night.

The Basics of Effective Greenhouse Light Management

Half-trays were placed in custom ebb-and-flood systems assembled in the environmental chambers. Nutrient solution reservoirs were crafted from commercial plastic storage tubs with a ~12.5 gallon capacity for nutrient solution. Reservoirs were filled with a solution of deionized water amended with a complete, balanced water-soluble fertilizer providing 200 ppm N (EC of 1.52 mS/cm). Trays were flooded each morning for 15 minutes beginning for the first two weeks after seeding, then an additional 15-minute daily flooding event in the evening was added for the last 14 days.

Four weeks after seed were sown, data were collected. The diameter and fresh weight of trimmed and washed radishes were recorded, and additional radishes were graded by hypocotyl diameter according to USDA grading standards. Additionally, leaf number, length, and greenness were recorded prior to trimming, to account for producers selling radishes with foliage in bunches.

Our Results

The responses of leaves and roots to temperature — while somewhat similar — were different. As air temperature increased (up to 73°F), radish diameter increased, after which radish diameter decreased as temperature continued to increase. We saw this same response for the fresh and dry weights of the roots, increasing with temperature up to 73°F and decreasing with warmer temperatures.

Although grown for their roots, leaves can also be important for marketing bunched radishes (a value-added approach). There was no difference in leaf greenness across air temperature, all having a healthy, green appearance. The number of leaves increased with temperature from 45°F to 65°F, but not above that. We suspect that at warmer temperatures, the radishes had already initiated flowers, which are terminal and would have stopped new leaves from forming. The fresh and dry weight of foliage increased as temperatures increased nearly all the way up to 82°F, with smaller leaves formed at 91°F.

Managing Temperature for Production

We saw no cold damage on either cultivar at cool or cold air temperatures. Radishes are already well-known as a cool-growing crop, and the results of our research certainly support this. However, what was a bit more surprising was to see optimal temperatures for root and shoot growth at 73°F and 82°F, respectively. So the first question you may be wondering is: “If radishes can tolerate cool temperatures, why would you produce them under warmer temperatures?” It all comes down to crop time. Yes, radishes certainly tolerate cold temperatures, but how long does it take to finish them when grown cool? If we use ¼ oz. as a target radish weight (a high-quality radish with the USDA Grade of “Large”), it takes just about four weeks to reach this size when grown at 65°F.

If the same crop were grown at 57°F, it takes approximately twice the amount of time to get the radishes to the same size. This longer crop time will result in greater heating costs, coupled with fewer crop turns, compared to growing at a warmer temperature.

In our study, we did not see an effect of temperature on the quality of the radishes, with respect to appearance and taste. The percentage of roots that would be classified as “damaged” according to the USDA Grades did not change with air temperature. Also, although we did not do any formal taste-testing with a panel, we did not notice radishes grown in temperatures up to 73°F in our informal sampling.

The Takeaway

Radishes may be cool, but their potential is hot! If you are interested in growing radishes in a controlled environment, challenge the perception that they need to be grown cold. When the faster growth rate at warmer temperature is taken together with their high planting density, there is great potential for radishes as a fast-turning food crop for controlled environments.

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