Late Summer Lettuce Transition

We are right now on our cusp for switching from sowing summer lettuce varieties to sowing cold-hardy ones. August 12 is my last date for sowing heat-resistant lettuces (mostly Batavians, plus Jericho romaine). August 16 is my first date for sowing fall varieties. Many of these are ones we grew in spring: Bronze Arrow, Green Forest romaine, Kalura romaine, Nancy bibb, New Red Fire, Oscarde, Panisse, Pirat, the Salad Bowls, Skyphos bibb, Starfighter, Swordleaf (an unusual lettuce that goes in and out of favor here).

From August 16 to 26 (sowings #29-32) I will sow less cold-hardy fall and spring varieties, then switch to the ones we find best for cold-hardiness: Buckley, Ezrilla, Hampton, Merlot, Red Tinged Winter, Revolution and a few others.

I get a break from sowing lettuce from September 1 to 11. Since we put up our hoophouse in 2003, we no longer grow lettuce in our coldframes over the winter. Lettuce does really well in the hoophouse, and spinach does better than lettuce in the coldframes, so we grow that there. I start again September 13 and these lettuces will be transplanted into our frost-free greenhouse to harvest as leaves until March, when we need the compost they are growing in, and the window space, for our flats of spring seedlings. In mid-September and a week or so later, I sow cold-hardy varieties to plant in our hoophouse and harvest as leaves all winter.

The Batavian varieties are cold-hardy as well as heat-tolerant, but we have been eating Batavians only (plus Jericho) for months and are ready for some diversity. This last week of summer sowings I only have a choice of about 6 Batavian varieties left (plus Jericho). I sow 4 different varieties each time to give us some range of color, shape and texture.

Note that the table is spread over 4 pages, so click the arrow to navigate. There are also buttons to enlarge the words.

Because daylight is getting progressively shorter and temperatures are getting colder as fall arrives, the time it takes lettuce to mature gets longer. To keep up a regular supply we sow every 3 days in late August, and every 2 days in September. Sowing a day later in fall can lead to a harvest being a week later, or more. We stop sowing lettuce on September 24, unless we have a disaster and need to sow some fast-maturing ones to try to make up for lost time. We won’t be harvesting those as heads, so we won’t worry about keeping up a regular supply once it gets wintry. The regular supply will come from those same plants growing new leaves. That works so much better than sowing seeds and starting from scratch, growing a whole plant.

Not much has changed on our choices of lettuce varieties since last year, except that we did use Tokyo bekana as a hot weather lettuce substitute when some sowings failed to germinate. I station-sowed it, that is, I put 3 seeds directly in the ground where I wanted the plant to grow and after emergence I pinched off the extra ones, to leave the best to mature. They grew well, and we had more than enough. This year, I have again resorted to Tokyo bekana, this time sowing it alongside lettuce seeds in the nursery seedbed, for transplanting in a couple of weeks.

Another potential change is that we noticed the leaf stems of the romaines got frozen, so we might skip romaines this winter. Or tuck them in surrounded by big leafy lettuces, to hide them from Jack Frost. We also talked about putting up a short length of snow fence type material to protect the crops in the ends of the central beds from cold when the doors are open in winter.

Once hardened to cold temperatures, most romaine varieties can withstand a couple of hard frosts, and survive temperatures as low as 20°F (-7°C). Cold-adapted varieties survive lower temperatures. But, if not sufficiently hardened, at 25°F (-4°C) romaines can have ice form in the stems, bursting the cell walls and turning to mush when they thaw. When frozen, the layers of cells lift away from the underlying tissue, leaving a puffy appearance.

The National Weather Service has three categories of cold temperatures above extreme cold, but nonetheless freezing: frost, freeze and hard freeze.

• Frost: frost is frozen dew. The droplets on the ground, on leaves and other surfaces become frozen whenever temperatures fall into the mid-30s and the winds are calm. A long frost may be referred to as a “killing frost,” even though not all crops die at the same temperature.
• Freeze: denotes temperatures are at or below 32°F (0°C) and other conditions, such as wind, prevent the formation of frost.
• A hard freeze happens when temperatures drop to 28°F (–2°C) or lower for more than four consecutive hours.
• A killing freeze occurs when temperatures are at or below a critical temperature for a period long enough to kill off crops. The exact temperature depends on the crop. See my table of Winter-Kill Temperatures of Cold-Hardy Vegetables

I wrote about our Fall Lettuce Transition in my post Preparing your Hoophouse for Fall and Winter 9/28/22.

That post includes days to germination of lettuce at various soil temperatures. Here’s a reminder: The soil temperature range for germination of lettuce seeds is 35°F -85°F (2°C -29°C), with 40°F -80°F (4°C -27°C) being the optimum range and 75°F (24°C) the ideal. At 41°F (5°C) lettuce takes 15 days to germinate; at 50°F (10°C) it takes 7 days; at 59°F (15°C) 4 days; at 68°F (20°C) only 2.5 days; at 77°F (25°C) 2.2 days. Then the time to germination increases to 2.6 days at 86°F (30°C). After that it’s too hot: Sow in an airconditioned space, a refrigerator, or use ice cubes and shadecloth.

Here is a list of our “stop-dates” for sowing lettuce for growing in various places (not just our hoophouse).

• 8/29 last date for sowing for outdoor row-covered lettuce to harvest 12/10-12/31.
• 9/9 last date for sowing to transplant in coldframes and harvesting leaves all winter.
• 9/21 last date for sowing for planting in an unheated frost-free greenhouse to harvest by the leaf all winter until March.
• 9/24 last date for sowing to transplant in our double-layered hoophouse to feed us all winter, 11/16-3/1, as we simply harvest the outer leaves, rather than cut the head.
• 11/9 last date for sowing “filler leaf lettuces” in our hoophouse to replace casualties.
• 1/25 is our last date for filling any gaps in the hoophouse beds with filler lettuce plants. After that, we fill all gaps with spinach plants.
• 2/15 last date for sowing baby lettuce mix in the hoophouse for harvest from mid-March to May, when it gets too hot. By then we should be happily harvesting juicy lettuce heads outdoors and will have lost interest in the lettuce mix. We like Fedco’s 2981 Lettuce Blend Organic or Johnny’s Allstar Gourmet Lettuce Mix #2301. For those with challenging growing conditions, both companies offer other specialized selected mixes. 1 oz (28gm) of seed sows about 600 ft, (200m)

This list comes from my post Winter Lettuce and Other Salad Crops