There's nothing better than the taste of home-grown spinach. Unfortunately, many gardeners who plant in spring are disappointed by meager harvests of small, tough leaves from plants that bolt (go quickly to the flowering stage) before producing large, juicy leaves.
If your spring-planted spinach never produces well, now is the time to plant it as a fall crop instead. Why? Because spinach is a cool-season crop that does best when days are less than 14 hours long and temperatures don't exceed 80oF. The best available time to get these conditions, especially in cold-winter areas (USDA Hardiness Zones 6 and colder), is to plant at the end of summer for a fall harvest. To determine the best planting time and varieties to use, I sowed several leading home garden varieties on different late-summer dates and compared notes with gardeners across the country following the same schedule. Here's what we found.
Except in the South and Southwest (zones 7 and warmer), spinach has traditionally been planted in spring for harvest in early summer. But in these zones, it's sown from early October to mid-November for winter harvest starting six weeks later.
Two major problems with getting spring-planted spinach to grow well are day length and heat. While heat is usually thought of as the main cause of bolting, day length actually initiates this flowering response in spinach. Flowering in most varieties used in North America is "switched on" by days about 14 hours long--as early as the middle of May for many areas north of the 36 degrees latitude (roughly a line running from Washington DC to San Francisco).
By the time warm spring weather arrives and the spinach is growing nicely, the days have become long enough to initiate flowering. Then days above 80° F speed up the plants' metabolism, accelerating the rate of bolting.
The solution is to plant a fall crop. For fall harvest in cold-winter areas, gardening experts usually suggest planting four to six weeks before a killing frost. This advice, however, is best suited to growing spinach to overwinter for an early-spring harvest (increasingly the case in Northern areas), not for a bountiful fall harvest. Chris Blanchard, a market farmer at Rock Spring Farm near Decorah, Iowa, plants his overwintering spinach between September 1 and 10 to get plants 5 to 6 inches in diameter by the time cold stops the growth."That's the perfect size for overwintering, but doesn't yield much spinach for a fall harvest," he says.
For a fall crop in my garden in coastal Washington state (zone 8), I decided to try planting earlier. I planted on August 1 and 15, 8 and 10 weeks, respectively, before our usual first frost in mid-October. For comparison I also planted on September 1, the usual sowing date for overwintering spinach crops in my climate. To find out if these earlier planting dates would work in other cold-winter areas, I enlisted David Cavagnaro, a horticultural photographer in Decorah, Iowa (zone 4), and Charlie Nardozzi, senior horticulturist at National Gardening in Burlington, Vermont (zone 4), to augment my plantings in Bellingham.
Once I had decided on the planting dates, I needed to pick the varieties. I chose a wide range including the savoyed (crinkly-leafed) 'Long Standing Bloomsdale' and 'Winter Bloomsdale', smooth-leafed 'Olympia' and 'Space', and the semi-savoyed 'Tyee'. On each planting date, we all planted 2- to 4-foot-long rows of each of these varieties.
Planting conditions is all three sites were similar. In general, fall spinach can be successfully grown on any fertile, well-drained soil with a pH between 6 and 7. It's best to plant in sandy loam in raised beds to keep the soil warmer and well drained, especially once the weather turns cold. Compost is an excellent soil amendment, providing a steady supply of nitrogen for luxuriant growth.
Planting in August was a bigger success in all three locations than any of us had imagined. Even though we all had hot weather in August and early September, yields were terrific, with several varieties producing succulent leaves as big as dessert plates by October.
A number of surprises occurred with the seven varieties. 'Tyee' was the biggest surprise. It is the standard variety for spring sowing because of its ability to resist bolting in early summer, but 'Olympia' usually outperforms it in spring. However, the tables were turned in both of our August plantings, with 'Tyee' clearly outperforming these varieties. It yielded delicate baby leaves by the end of August and full-sized ones by mid-September. Tied for second place were 'Olympia', 'Indian Summer', and 'Coho'.
With cooler temperatures by mid-September, the August plantings of 'Olympia' and 'Space' started to really grow in Washington and Iowa. While the growth rates of both 'Tyee' and 'Indian Summer' started to subside for us, they stayed the top performers in northern Vermont, with its more extreme fall weather. At this time, Charlie and I noticed the August 1 plantings of 'Coho' and 'Indian Summer' starting to bolt, while in David's garden the August 1 planting of 'Long Standing Bloomsdale' bolted. This may have been due to variety differences and day length being at or above 14 hours in all three locations until August 15.
By the end of September, our August plantings were producing lots of leaves The last surprise came in October for David and me after fall's real chill started to fill the air. While the growth rates of all other varieties slowed down seriously, the August 15 sowing of 'Coho' kept on growing, becoming the top-yielding variety in October. We also noticed that the dependable 'Winter Bloomsdale' was still putting on some cold-weather growth; its beautiful dark green color also stood out. The September 1 plantings of all varieties never really produced leaves as large and as numerous as the August plantings and, as indicated earlier, seemed to be better for overwintering rather than fall harvest.
Overall, the best planting date for all of us was August 15. Our August 1 plantings, while good for some varieties, did produce some bolting spinach. All of our September 1 plantings grew respectably, giving us silver dollar-size baby leaves, but not like the yields of our August plantings.
Amazingly, we all had similar results with the varieties. The most successful ones for August growing conditions were 'Indian Summer' and 'Tyee'. With cooler conditions in September, the smooth-leafed varieties such as 'Olympia' and 'Space' started to shine. The cold winds of autumn showed us how well 'Coho' could grow--right until December in my Northwest climate. So, for a continual fall harvest of delicious spinach in a northern area, try a combination of these high-performance varieties.
Dr. John Navazio is a vegetable breeder in Bellingham, Washington.
Photographs by David Cavagnaro.
Article published on June 23, 2008.