In the late summer of 2010, High Mowing constructed a high tunnel with the goal of extending our growing season and conducting overwintering variety trials. We received a grant from the Natural Resources Conservation Service (NRCS) to put up a high tunnel through their Seasonal High Tunnel Initiative. For the past two winters we’ve conducted variety trials on overwintered crops – on their yield, flavor and cold hardiness. We’ve planted several different crops in the high tunnel – mustard and Asian greens (for baby leaf and bunching), kale, pac choys, and mini head lettuce . But the majority of what we’ve planted in the high tunnel is spinach, and that’s what we will focus on here. Prepping the Beds In mid/late August we begin prepping the high tunnel for winter crops. This involves taking out the warm season crops (this year we put in cucumbers and okra), irrigating the soil, letting it dry out to a good moisture level for tilling, adding compost, and roto-tilling (we use a walk-behind Troybilt tiller). Once the soil is prepped, we use a measuring tape and string to mark out our beds. Our beds in the high tunnel are 4’ wide. This is wider than our beds in the field because we want to maximize our growing space by minimizing the number of paths. Paths in the tunnel are about a foot wide. We form beds by shoveling 3-4” of soil from the paths onto where the beds will go, then raking out the mounded soil to create an even planting surface. Seeding the Crop We direct seed the spinach using a 6-row precision seeder. In a 4’ bed, we seed 9 rows about 4.5 inches apart. We obtain this row spacing by only filling every other hopper in the seeder and then doing three passes over the bed. We water in the seed by hand using a garden hose with a wand attachment. When to Plant In 2010, we seeded spinach in mid-September. We starting harvesting in mid-November and most varieties were harvested a total of seven times, through late April. A few of the varieties we harvested into May for a total of nine harvests. In 2011, we direct seeded spinach in late September, then again the first week of October. This difference of one week seeding time didn’t ultimately seem to have much of an impact; the varieties seeded in early October caught up. Both plantings were ready to harvest shortly before Christmas. A few of the varieties we seeded in late September had poor germination, and it wasn’t until mid-October that we realized the problem and got around to reseeding. These varieties, seeded by hand in mid-October, did not mature and did not catch up to the earlier plantings, so in our region, the difference between an early October and a mid-October seeding date seemed substantial – at least it was last fall. We experienced this first hand, but Eliot Coleman has a great chart in The Winter Harvest Handbook to save you a little bit of trial and error. The chart shows that a crop with 40 days to maturity, planted on 9/19 would be ready for harvest about 70 days later, on 11/25. But that same crop, planted on 10/17, would take 110 days to mature, not ready until 2/3. This year, we plan to experiment with seeding spinach every week from early September through early October to try to pinpoint our optimal seeding date, and how different seeding dates affect maturity and yield for our area. Maintaining the Crop Once the spinach gets established, we usually weed once or twice before the weather turns cold. Once that happens, the weed pressure drops considerably. We’ve found that irrigation isn’t necessary. The cool temperatures help retain moisture in the soil, the plants aren’t growing as fast or drawing as much moisture, and condensation in the air keeps things generally moist. Last year, because we had a dry, mild winter, we found that we had to begin irrigating pretty early in the spring – starting in late February – which hadn’t been the case the previous year. This ended up being a considerable amount of work as we thawed out hydrants and dragged frozen hoses around. Irrigation is a good thing to think about and plan for when you’re setting up your high tunnel. We cover the beds with wire hoops and row cover for a second layer of protection/insulation starting when the outside temperatures drop to around 25 degrees F. When we want to harvest or uncover the beds for venting, we walk down one side of the bed, picking up the edge of the row cover and tossing it over the bed into the path on the other side. Once we get to the end of the bed, the entire length of row cover is loosely folded, length-wise, along the bed in the adjacent path. To recover the bed, we again walk the length of the empty pathway, this time pulling the edge of the row cover over the bed as we go. For the past two seasons, we’ve used a double layer of AG-19 (standard weight) row-cover over each bed. This year, we’re going to experiment with using just one single layer of AG-19. This is because, after rereading parts of The Winter Harvest Handbook, we saw that Eliot Coleman trialed light-weight (85% light transmission) and heavy-weight (50% light transmission) row covers, and found that the crops actually did better under the light-weight fabric because it allowed for more solar gain through greater light transmission yielding warmer temperatures. Presumably, with our double layer of AG-19, the light transmission would be about 70%, so better than the heavy-weight row cover, but still, it might be inhibiting some warming. In late February to mid-March we start watching for overheating under the row cover and removing it during the day on warm or sunny days. Harvesting Both years, we got one harvest off the spinach before Christmas (early November for the 2010 mid-September seeding date and mid/late December for the 2011 late September/early October seeding date), then no harvest again until February. In our region of northern Vermont, early February is when the days lengthen to over 10 hours and plants begin to emerge from their semi-dormancy. But you can see that the earlier seeding date resulted in two earlier harvests, each by two to three weeks, which is significant.


2010-11:    (1st) 15-Sep    (2nd) 9-Nov    (3rd) 9-Feb    (4th) 18-Mar 2011-12:     (1st) 29-Sep   (2nd)22-Dec    (3rd) 26-Feb  (4th) 14-Mar

By the time of the third harvest, in mid-March, the plantings had pretty much caught up with each other. From mid-March through early May, the regrowth is steady enough to harvest about every 10 days. We choose to harvest leaves individually by hand, because we’ve found that when we clear-cut the planting the regrowth is much rougher. Yield In our 2011 trials, the highest yielding variety, Pigeon F1, produced 1.75 lbs per square foot planted for a total of 35 lbs. The average was 1.5 lbs per square foot with a total of 28 pounds. So, if you figure that a high tunnel this size has about 1600 square feet of plantable space (minus the paths), at 1.5 lbs per square foot that equals 2400 lbs total yield over the course of the season. Our lowest yielding variety in the trial, Samish F1, was a little over 1 lb per square foot – so varieties can make a difference in the total performance of your high tunnel. It’s a big investment of time and money, so make sure you’re getting out of it as much as you can! (Renegade F1 and Giant Winter were also high yielders in our trial.) Where to Get More Information This winter will be our third season producing winter crops in a high tunnel, and there are certainly many growers out there who have been doing this for a lot longer than we have. We’re always interested in hearing how things work for you so please keep us posted on your high tunnel winter growing! To name only a few of the many more experienced and knowledgeable growers: To help you determine day-lengths for your area, and when the day-length drops below ten hours: If you want to build a high tunnel, or looking for more resources about growing in them:
  • Useful information, links and articles regarding high tunnel vegetable production:
  • High Tunnels is a great manual from the University of Vermont: