When we surveyed Indiana high tunnel producers eight years ago, about two-thirds of the operations grew cool season crops for harvest from November to March. Although spring field planting season is just underway, it is not too early to plan for markets and organize a production schedule for fall. This article summarizes common crops and their planting periods and is written for those new to winter production.
Baby-leaf greens for use in salads or cooked dishes are widely grown in high tunnels. Examples include mizuna, mustards, Swiss chard, tatsoi, kale, bok choi, and lettuce. They are commonly direct seeded in dense plantings (Figure 1). Direct seed in the tunnel from mid-September through early November. For early plantings, the first harvest is in about 3 or 4 weeks. Harvest by cutting an inch or two above the soil using shears or a greens harvester. Crops with a rosette growth habit will have the growing point below the point of cutting and will regrow to be harvested a second, third, or possibly more times. The time to regrow will depend on the environmental conditions: faster with warmer temperatures. The different kinds can be planted in separate areas, and mixed together after harvesting if the product is mixed baby greens. Some growers may prefer to plant crops mixed together, and just harvest the mix. The drawback of this method is that different kinds and varieties of greens often grow at different speeds, and so the mix might not have the desired proportion and size of the different types of leaves.
Figure 1. Mustards were direct-seeded on September 29, and the photo was taken on October 28. Left to right, varieties Green Wave, Golden Frills, Giant Red, and Ruby Streaks (Photo by E. Maynard).
Spinach could also be classified as a baby-leaf green, although sometimes larger, older leaves are harvested. It may be direct seeded or transplanted, usually late August to early November (Figure 2). Very high soil temperature inhibits spinach germination so some prefer to transplant the earliest seeding rather than direct seed into hot soil. Because plants are normally spaced inches apart, the paper-pot transplanting system is especially helpful in reducing transplant labor (Figure 3). Typically, a single planting is harvested multiple times, with regrowth between harvests. Harvest can be done by clipping individual leaves (more labor intensive) or clipping above the growing point with shears or a greens harvester as for other baby-leaf crops. Entire plants can also harvested, but then there will be no regrowth. Spinach will routinely survive the winter, and harvest can continue into early spring. Earlier plantings, i.e., late August or early September, will produce more over the course of the crop than later plantings. Sometimes, multiple plantings from September to early November are planned to provide a steady supply of good quality leaves.
Figure 2. Spinach ‘Gazelle’ seeded October 3, photo January 2 (Photo by E. Maynard).
Figure 3. Paper pot transplanter saves labor, especially when transplanting closely-spaced seedlings (Photo by E. Maynard).
When leafy greens crops are harvested at a larger stage, for instance, full-size leaves of Swiss chard, kale, or collards, transplants are usually used to get a head start on production (Figure 4). Seed in trays about 4 weeks before transplanting into the high tunnel. Transplant kale and collards from early September to the first week of October, Swiss chard from mid-September to the first week of October, and mustard in October. A single planting of kale or collards can be harvested through the winter and into early spring. The earlier it is planted into the tunnel, the higher the yield will be. Swiss chard and mustard might not survive the winter, but earlier plantings will allow the most harvest before production is ended by extreme cold. These crops are typically harvested by removing leaves that have reached the desired size and then waiting for the next harvest until younger leaves grow (Figure 5). The speed of leaf growth will depend to a great extent on temperature.
Figure 4. Month-old kale seedlings ready for transplanting into high tunnel (Photo by EA Bluhm).
Figure 5. Kale cv. ‘Darkibor,’ February 2 (Photo by E. Maynard).
Lettuce can also be grown for harvest at a larger stage after a head has developed (Figure 6). Transplants are useful to get a head start in that situation. Seed transplants about 4 weeks before transplanting. Sequential plantings can be done every one or two weeks to supply a steady harvest, with a final fall transplant date by early November. Multi-leaf lettuce types have a growth habit that makes it easy to cut just above the growing point and obtain a handful of leaves, similar to a baby-leaf harvest. With this harvest method, the multi-leaf types will regrow and can be harvested more than once. However, regrowth will depend on temperature, and some prefer not to rely on the regrowth but instead move on to another planting.
Figure 6. Multi-leaf lettuce ‘Red Incised’ (left) and ‘Green Reef’ (right), transplanted October 14, photo December 5 (Photo by E. A. Bluhm).
Root crops represent another category commonly grown in high tunnels. Multiple seedings of radishes and salad turnips from mid-September through early November should provide harvests into January (Figure 7).
Figure 7. Radishes ‘Cherry Belle,’ seeded September 29, photo November 11 (Photo by E. Maynard).
The suggested planting dates above are for Hardiness Zone 6a, assume a high tunnel with a single layer of plastic, no supplemental heat or light, and row covers used to protect crops when temperatures drop below 25°F. In high tunnels where supplemental heat is provided crops could probably be planted a month later in fall, but we have not collected as much information under those conditions.
Of course, this is not the complete list of crops. The publication Scheduling Fall and Winter Vegetable Production in High Tunnels, HO-330 (https://www.extension.purdue.edu/extmedia/HO/HO-330-W.pdf) includes additional crops, as well as more information on planting schedules in general, and suggested February and March planting dates for spring harvests.
For any crop in the fall high tunnel, it is important to recognize that the bulk of growth will occur by mid-November. After that, shorter days, lower light levels, and cooler temperatures mean that growth is very slow. Therefore, the goal is to plant so the crop is near the harvestable stage by mid-November. In order for that to happen, it makes sense to start planning now.