How Plug Tray Density Affects Hydroponic Radish Production

Over the past two years, articles in this series have focused on the production of a range of vegetable crops grown in plug trays using flood and drain irrigation. Within the category of root crops, radish, carrot, beet, and turnip have been grown successfully from sow to harvest.

Exploring radish production further, how does plug tray density affect yield? A radish crop approximately halfway through its cycle on a flood and drain bench is pictured in Figure 1 (see slideshow). Three plug tray densities were used in the experiment: 72, 105, and 128.

Easier to observe, the three trays of plants are shown at maturity with the leaves removed in Figure 2 (see slideshow). Seeds were sown on October 10, 2014, and final data and harvest occurred on December 16. Declining natural light contributed to the extended crop time. As expected, an inverse relationship was observed between cell density (72, 105, 128) and radish size where lower cell count (bigger cells) produced larger radishes.

Following harvest, the radishes were washed and graded. Figure 3 (see slideshow) presents the yield distribution for the three plug trays. Each column contains the harvest from a single plug tray. Left to right they are 72, 105, and 128. Within each plug tray (column), radishes were graded into three sizes — large, medium, and small — from bottom to top, respectively.

Cell density affected the distribution of size with the largest cell size (72 tray), producing the most radishes of large grade (Figure 3, bottom left, see slideshow). Note the decline in large-grade yield (Figure 3, bottom row, see slideshow) as cell size decreased from left to right (72 to 105 to 128). Similarly, note the increase in medium- and small-grade yields from left to right and bottom to top.

The obvious conclusion is that increasing plug tray density (cell number) resulted in lower yield of traditional market-size radishes. Two points can be considered, however. First, the late fall crop cycle under natural light likely played a role in radish size variability within and between different tray densities. It is anticipated that repeating the experiment during higher-light months, as well as during the same cycle with supplemental light, will reduce crop time and yield variability.

Think Differently

The second point regarding radish size is intriguing. I took the radishes home and prepared them for inclusion in a dinner salad. Figure 4 (see slideshow) shows the three grades on a cutting board after prep. The traditional, market-size radishes are on the right, medium size in the center, and small grade on the left.

Most producers of radishes consider the pile of small radishes on the left as culls to be eliminated from the harvest and discarded. The center pile of medium-size radishes might also be thought of this way. How about consumers; do they see these piles similarly? Have we even bothered to ask them for their opinion?

A recurring observation throughout this work with high-density plug tray production of vegetable crops has been that smaller than traditionally sized produce may offer opportunities at several levels. This theme is consistent in the project and to date has been shown to apply to carrot, Pak choi, beets, turnips, and celery, where smaller harvest size may offer advantages. Shorter crop cycles and higher production densities are two, both contributing favorably to profitability.

From the consumer perspective, this may open the door to offering smaller, personal serving sizes. In the case of radish, we might offer three grades: slicer, splitter, and bite size. Bite size snack radishes could complement cherry tomatoes, snack-size peppers, and finger-sized cucumbers and carrots.

Optimizing Production

One goal of high-density production of any crop is to optimize uniformity in order to minimize grading. Much of the optimization effort deals with managing field, greenhouse, or indoor growing conditions. The title of this project, From Flowers to Food, assumes that many who will grow these crops in a greenhouse may have ornamental crop experience.

Once guidelines are set for optimized production, a grower would simply choose the plug tray density to yield the grade of radish desired. If he or she chooses to produce a traditional, large grade of radish, a 72 tray might be best. If medium or small grades are targeted, 105, 128, or smaller cell size will be used. Or, a grower might choose to produce more than one grade. His or her largest production block might be at a density of 72 cells with smaller blocks of higher densities to produce smaller, niche-market grades.

What About Demand?

In ornamental crop production, it is often true that per square foot revenue and profitability are correlated to crop density. As density rises, so too does gross revenue and almost always profit.

My experience with ornamental crops was that an 18-count tray (1801, 3.5-inch cell) supported retail pricing that produced revenue and profit margin on a square-foot basis that far exceeded 4.5 inch and larger container sizes. The rub was that as a retailer, I needed to have a range of sizes and price points.

I am on record making the statement that if we could sell 1801-sized units of seeded vine crops like cucumber and zucchini as garden transplants with a crop time of four weeks, year round, we would all retire in comfort. Unfortunately, sufficient demand for this category of crop production doesn’t exist.

Why can’t we assume a year-round demand for greenhouse radish production? Sure, there would be some seasonal fluctuation tied to outdoor field production, but salad ingredients are in demand every day of the year. And what’s so far-fetched about adding bite-sized radishes to a lunchbox that is already packed with cherry tomatoes and bite-sized peppers, cucumbers, and carrots?

Density Limitations

Throughout 20 years of researching Florel as a growth regulator of ornamental crops, I repeatedly asked myself a simple question while passing crops in the greenhouse with a sprayer of solution in tow: I wonder what would happen if …?

Remaining true to this philosophy, today’s article ends with Figure 5 (see slideshow) showing a radish trial in a 288-plug tray. For an olive-sized radish, this cell size fits the bulb itself perfectly. That said, it’s the shoot growth that is excessive at this density.

If we agree that a plant is a plant, consider how in recent decades we researched and learned how to optimize high-density plug production of ornamental crops. That effort resulted in quality plug production at a density as extreme as that in a 512 tray.

Another interesting point from Florel work is that ethylene treatment encourages increased mass of vegetative organs including leaves, stems, and roots. This will be a focus of upcoming experiments with not only radish but the entire list of root crops cited above. Perhaps some ethylene pressure will direct more of the plant’s energy to the root area, thereby reducing crop time, increasing yield, or both.

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