Growers who utilize hoop houses in their market gardening operation do so primarily because these structures enable them to harvest earlier, thereby obtaining higher prices for their produce. While there are many benefits associated with hoop house production, chief among them is earliness of harvest.
Several factors contribute to early harvest when growing in a hoop house, including: management of roll-up sides and row covers to achieve the fastest growth possible; raised beds covered with black plastic mulch to achieve a warm root environment; use of early maturing varieties; use of large (older) transplants and an early planting date.
All vegetable transplants used in our hoop house research and demonstration projects are grown on campus in our research greenhouse. All transplants, with the exception of tomato, are grown in 36-cell seedling trays (2-inch by 2-inch per cell). Tomato transplants are grown in 4.5-inch diameter containers in order to take a larger, more mature transplant to the hoop house.
For the past seven years, we've operated under the assumption that larger transplants translate into earlier yields, but have not had any hard evidence that this is the case. Does increasing transplant size guarantee earlier harvests?
A study was conducted in one of our hoop houses during the spring of 2002 to determine the effect of transplant size (age) and planting date on early yield of "Road Runner" tomato.
A 20-foot by 30-foot section on one end of the house was utilized for this study. Treatments consisted of five different seeding dates and container size combinations (Table 1). Transplants were set into four 40-inch wide beds equipped with drip irrigation and black plastic mulch. Plants were spaced 24 inches apart in the row with one row per bed. Each treatment consisted of 12 plants, three plants per replication. Preplant fertilizer was applied according to soil test results. Supplemental nitrogen was applied weekly via the drip system based on a fertigation schedule developed for small plot fertilization. (See Permanent Raised Bed Gardening Vol. II, publication number NF-HO-99-12, pp. 25-26 for schedule details.) Wire cages supported the plants. A heavy-weight row cover was installed on March 21 to protect plants from a hard freeze (24 F) that occurred on the morning of March 22.
At the time of the March 13 planting, the 6-inch container transplants seeded on Jan. 21 had a well-developed root system. Plant height averaged around 18 inches, and some of the plants were in bloom. By contrast, the 6-inch container transplants seeded on Feb. 6 were smaller and the roots had not permeated the growing medium sufficiently to prevent the root balls from breaking apart when removed from the containers. The 4.5-inch container transplants seeded on Feb. 6 fared much better than the 6-inch container transplants sown on the same date in terms of ease of transplanting. The smaller container size assured a more cohesive root ball that didn't fracture during planting.
Harvest began on May 13 and concluded on July 10, by which time yield had diminished to a point that continued harvest was deemed unjustifiable.
Table 1 summarizes the performance of all five treatments. At a 90 percent confidence level, there was no significant difference in early yield among any of the early-planted treatments. The 4.5-inch container transplants produced as much early yield as either of the 6-inch treatments. As expected, both of the late-planted treatments produced significantly less early yield compared to the early-planted treatments.
The difference in total yield among treatments is most likely associated with a difference in plant maturity. When harvest diminished to a point where only a few fruit were being harvested from the early-planted treatments, we decided to conclude the study. It's fair to say the difference in total yield between early and late-planted treatments would have been reduced if harvest had been continued for another couple of weeks. Even so, if the real goal is early yield, it doesn't matter.
Compared to 6-inch container transplants, 4.5-inch container transplants are less expensive to grow and purchase, are more efficient to transport and easier to plant.
Based on the results of this study, the use of 4.5-inch container transplants seeded early enough to ensure a well-developed root system and planted in mid-March is the best bet from a cost/benefit perspective for producing early-season hoop house tomatoes in southern Oklahoma. Growers in other areas of the country, or with the ability to provide supplemental heat, will need to adjust seeding and transplanting dates accordingly.