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How To Grow Yourself Some Lettuce Seed  

Frank Morton

If you want to try seed growing generally, or if you want to specifically grow a seed that is easy, valuable, and a worthwhile contribution to your food security,
Lactuca sativa (“lettuce of cultivation”) is a very good place to begin.

Most growers have seen the beginnings of lettuce coming of age. When the stem begins to elongate we know that it’s salad game over, the goodness has bolted. If you live in a warm dry place, and if you started your lettuce in March or April, you may have seen the whole life cycle play out. The stem develops a flowering head of small yellow blossoms, each blossom lasts a day, and is followed in 2-3 weeks by a dandelion-like puff of fluff, each bit of pappus attached to a single seed by a fine filament. Every yellow blossom matures in this way, and after 2 months of flowering the entire head is one large white aggregation of seed capsules, each blossom now represented by a small cup holding 12-20 white or black seeds, each with a would-be ticket to ride the wind. If you are this lucky, you could collect that seed at that moment.

For most of us, growing lettuce for seed needs to be more intentional than the above. If you live in a humid climate, summer rain will make a mess of that seed head as it matures. If your growing season is short, or if your lettuce started later in the season, the seed may never mature. If deer bite off the bolting stems, which they relish, the regrowth will mature a month behind schedule. If the maturing plant is blighted by stem rotting bacteria or fungi, it may die before setting or ripening seed. If a period of rain arrives just as the seed is maturing, the entire head may dissolve into a grey fuzzy fungus, or the seeds may sprout while sitting in little cups of water. Or, if everything else goes right, and you are just waiting for the last of the seeds to ripen, goldfinches may arrive hungry on their southerly migration, and consume every seed in an afternoon. There are a lot of opportunities for failure in any seed growing endeavor, and you only get one opportunity per year to learn all the ways. But really, except for all of this, lettuce seed is easy. That was a joke.

The proper planting time and growing situation is entirely dependent on your location. Most commercial lettuce is grown for seed in arid climates, like the western United States. If grown in the humid eastern US, it is very advantageous if the ripening period in late summer is generally dry. The ripening heads can be rained on without damage if there are drying periods between storms. If prolonged wet spells are to be expected during the ripening season, growing under plastic tunnels with sidewalls rolled up is the best plan.

Planting time is probably earlier than you think. Because our heat units accumulate so slowly in western Oregon, and because my target dry spell is August, I plant no later than February 20th for a successful harvest, a month before I start peppers and basil. Summercrisp and crisphead
varieties are the slowest to mature, leafy types generally fastest, with romaine and butterheads mixed up in the middle. Every variety has its own number of heat units to maturity. In the hot and humid East, I suspect seeding in April would give the same maturity date as ours.

How many seed plants do you need? One lettuce plant may produce 25,000 seeds, plus or minus 25,000. A real deal, and more than many people need for themselves. But if you only grow one plant for seed there’s a real risk you might get nothing, because as mentioned before, many challenges are to come between the market-ready salad head and the mature seed head. I like to start with a minimum of 27 plants of a variety, which allows selection of some “best performers” and some to eat. I select my favorites by putting a small stick on the north side of each one I prefer. Strangely, it happens too often that these sticks are like curses, especially for the very favorite one. They really do die; of bacterial wilt, or sclerotinia, or some damned virus vectored by aphids, or a gopher, or ground squirrels. This is why you need to start with enough plants, so that all the whittling down by you and nature still leaves you with a “best performer” to produce seed.

In many other crops, one or several plants is never enough to maintain an essential level of genetic diversity in the crop population. This is the case for all the normally outcrossing species; corn, brassica, carrots (and all umbels), beets, onions, radish, spinach, etc. The natural inbreeders, like lettuce, beans, peas, wheat, and sweet peppers, can be selected down to a limited parentage (indeed, to a single parent) without the phenomenon of “inbreeding depression” coming into play. This “depression” of plant vigor and fecundity isn’t well understood on the genetic/physiological level, but it is well documented and has frustrated many seed savers of corn and cabbage. The natural inbreeders are “used to” this genetic bottlenecking, an evolutionary adaptation allowing individual preadapted plants to colonize large uniform areas without any associate pollinators. Dandelions are an extreme example of this, going beyond self pollination to apomixis, with windblown seeds formed by genetic cloning of unfertilized ovules. Outcrossing crops, in contrast, require from 60-300 pollinating individuals in a population to keep the population vigorous and reproductive, and they need a means to cross pollinate. Seed savers really do need to consider this as they steward their outcrossing crops.

Final plant spacing for lettuce seed plants should be at least 12” apart in the row, rows at least 24” apart. Wider spacing allows better airflow for better plant health. Lettuce seed plants can take dramatically differing forms as they mature, varying by individual and by variety. Many of the heirlooms and old commercial standards are large branching plants, nearly bushes in some cases (‘Red Sails’ red leaf and ‘Waldmann’s Green Leaf’ come to mind), while lettuces of the modern era tend to be tidy, single stemmed, unbranched, short-statured seed plants. Those big old style plants could produce ounces of seed each, but are difficult to harvest and prone to late-stage diseases. The more compact single-head style plant produces an ounce or less of seed, but are efficient to harvest and manage in the field. Some growers use stakes and string to trellis the bolting lettuce rows, especially where wind is a factor. There’s a tendency toward top-heavy floppiness as the seedhead fills, especially if rain is weighting them. Once the plants begin flopping over, this invites more dampness and mold because they don’t dry out thoroughly in the sun and wind.

Selection of the best plants doesn’t happen in a single observation. I’ve counted as many as a dozen selection opportunities from seedling to seed in lettuce. If I sow three seeds per cell and germinate them in February in an unheated greenhouse, the first to germinate in each cell has something I want. If dark pigments are important, seeing those pigments in cotyledons or first leaves is also something I probably want. So, thinning flats that were sown 3-times too thick is a selection event if you pay attention. If mildew or aphids come to call on the greenhouse flats, that can be a selection opportunity. When the same challenges visit in the field, that is a selection opportunity. Which plants grow fastest, which plants get biggest before bolting, which taste the best, which have the thickest leaves, the glossiest coat, the deepest color saturation? Which reds are more red, less brune? Does it head up into a blanched heart? Is the heart tipburned (a very big deal)? Does summer heat cook the heart? Does the bolting stem grow straight up, or does it want to snake sideways (very bad). Is the bolting plant a tidy single stem or a branching beast? Does it form a symmetrical productive seedhead? Does the maturing plant maintain a healthy stem and base, or are fungi feeding there? Are there virus symptoms? Are there aphids and/or caterpillars in the seedhead (usually, yes)? Did the plant bolt and make seed early (not the best)? Did it bolt very late, so that the seed will scarcely mature (also a problem)? Which plants made the most and the best seed? This last item can be a very good overall assessment of mature plant growth and vigor. Where most people would be most focused on the look of the salad head on harvest day, a seedkeeper will be focused on overall plant health, up to the drying day. It is the vitality of the entire lifecycle of the plant that gives the best sense of adaptive fitness for the long run. I think.

Ripening of the lettuce seed is an accumulative process. There is no one perfect time that allows harvest of the entire yield potential at once. The seed industry has two basic approaches to lettuce seed harvest that are instructive. The “shake method” is the original way, the very best method to get the highest yield, best quality, and cleanest seed sorted by harvest into quality grades. As the name suggests, when the ripest seed begins to fall from the earliest pollinations, seed heads are leaned over/into bins/bags/buckets and shaken vigorously to collect fully ripened seed from the head. Care must be taken not to damage the unripened portions of the seedhead, or to fling seed in all directions. This labor-intensive process (plant by plant across acreage!) is repeated at intervals determined by imminent seedfall, as before. Ninety percent of the yield potential can be harvested in three shakes of the lettuce heads. The other harvest approach is the “one cut method,” when you wait a bit after the first seedfall, then cut whole plants onto fabric in the middle of the yield curve. Plants are threshed after 7-10 days of after-ripening, preferably in shade. This is less work, for less yield, of a lower quality. My own alternative to the one cut method is the “one pull method,” where the plants are pulled with the roots (rather than cut), so they remain alive nearly a week, maturing longer than severed plants. This improves both yield and germination over the cutting method. Another alternative (especially useful for trellised plants), is the “head-cut method,” where seedheads are snipped from the stems at optimal maturity and threshed at once.

After seed harvest, the seed is moist, metabolizing, and full of insects. It is important to spread the fresh harvest out to dry in a shaded, breezy location, so that aphids, spiders, lady beetles, lygus and minute pirate bugs, micro wasps, and syrphid larvae can crawl out, which they will. After drying several days or a week, when all the creatures have evacuated and the leafy chaff is drying out, you may begin the cleaning process by rough winnowing in a soft breeze outdoors. Start this by rubbing handfuls of seed/fluff over a Rubbermaid bin in a gentle breeze. All of the seed should fall into the bin, much of the fluff will drift away. Repeat this. Now put the seed/leaf/stem mixture onto a 1/8” hardware cloth screen. The seeds and fine chaff will pass through. Winnow the seed and fine chaff in a breeze again, but this time with two bins side-by-side, so the seed falls into the upwind bin, and light chaff, dust or any mistakes fall into the downwind bin.

Winnowing, mentioned in passing above as if well understood, is an entire subject unto itself. It refers to using the wind to carry away light, low density particles, from a falling stream of relatively heavy, high density seed. I mention using the wind (as opposed to a fan) because that’s my preferred way to winnow lettuce—outdoors in the wind. Indoors with a fan is no good, possibly deadly over time, due to fine dust. The proper way to wind-winnow is to use a square edged container, something like a bread pan, 4 gallon bucket, or bin, to pour seed toward/into the wind so that it all falls into a bin 12-24” below. There is a side to side shaking motion of the pan or bucket or bin that delivers the seed/chaff mixture in a steady falling curtain that the air blows through, carrying away the fine leaf particles and dust, cleansing the seed that drops heavily. This is to be repeated three or more times, the collected rewards becoming more and more purified with each passage through the wind. Once fine dust has been removed, a fan or other winnowing device can be safely employed indoors for more accurate winnowing, after screening the seed.

Screening and winnowing are used in alternating cycles to get the seed very clean. After winnowing away fines and dust that will clog up screens and are bad to breathe, lettuce seed can be passed through window screen (1/16th inch, square) with some determined, careful shaking, and maybe “bouncing.” Sticks and flower parts will stay on top as the lettuce passes through. Winnow again this screened seed. If you then pass the seed quickly back over the same screen without much shaking, fine seed, heavy dirt and “crippy crap” will bottom out, leaving the bulk of the crop atop the screen fairly pristine. This is your prize.

This is the basic lettuce seed-to-seed process. There are variations and options, many more ways to complicate matters by collecting packets of single mothers for future comparison, or selections of unique traits, or “off-types” that may actually be cool crosses. Lettuce was for me an inviting gateway to the world of seedkeeping.

Frank in a field with lettuce plants going to seed.

Originally published in the 2022 Wild Garden Seed Catalog.

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