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Lettuce- Bolting Resistance

Lettuce bolting study planting successions


Bolting (flower initiation) in lettuce is a common problem for producers in areas where summer temperatures rise above 85°F . Colorado’s Front Range is not known as a preferred lettuce production area; however there are growers that successfully produce lettuce throughout the summer months. While good production in the summer months requires special attention to production techniques, some varieties of lettuce resist bolting better than others. In this initial study, 50 varieties have been planted every two weeks, starting in June, and ending in September to evaluate variety responses, and to develop a protocol for expanded studies in the following years.

The varieties that we worked with were those provided by seed companies, some of which are currently grown here, and others which are not. All production practices have followed organic guidelines.

There is also interest in the health and nutritional quality of lettuce grown in Colorado, specifically those with darker red pigments. The Department of Food Science and Human Nutrition at Colorado State University is collaborating with the Specialty Crops Program to study the antioxidant and Vitamin E levels in the different lettuce varieties.


      1. To test 50 different varieties of lettuce over the growing season for their bolting resistance.
      2. To test the levels of antioxidant and Vitamin E levels in the red versus green varieties of lettuce.



The lettuce was sown in the green house into 72 cell trays, and grown-out there for about 30 days. Sowing was done every 2 weeks. Seedling emergence and vigor was very different between varieties. One seed source had consistently poor germination which suggested old seed.

The field where the lettuce was grown had been in alfalfa for the last three years. Poultry compost was applied and the field was roto-tilled, bedded to 30 inches and rolled. Individual beds were pre-irrigated (drip tape) about a week prior to transplanting to germinate weed seed, and then blind cultivated. The lettuce transplants were then hand planted at 8 inches in row for all varieties except the Romaine varieties, which were planted at 10 inches in row. Two rows were planted per bed. Early lettuce succession with drip tape

Weed control was minimal with one or two light hand weedings on the bed, and 1 or 2 hoeings in the furrows.

Pests and disease

Only one application of Bt was required for an early infestation of alfalfa webworm caterpillars. Subsequently nearly all caterpillars have been successfully controlled by natural enemies.

Flea beetles were damaging in the first two plantings, and no control was attempted. Some varieties were more susceptible than others, and general notes on damage were made.

Leafhoppers were very abundant during the first part of the summer. Significant numbers of plants were killed by a phytoplasm that is presumed to be carried by the leafhoppers. Again there appeared to be strong varietal resistance to either the leafhopper or the disease. No control efforts were made.
Diseased lettuce plant

Aphids were present at levels that commercial production would have controlled; however naturally occurring biological enemies controlled them before significant plant damage occurred. Subsequent plantings had few aphids. Two varieties have been found to have red aphid but not until September, and these are complemented with a variety of predators. Predators, rather than parasites have been the primary control agents observed.

Rabbits caused some damage, preferring the butter type lettuce transplants.



This summer’s weather was unusually hot and dry. We experienced 26 days above 90°F. Only one light hail event occurred, without significant impact.

High and Low Temperatures at HRC, 6/1/2003 – 9/14/2003

Graph of High and Low temperatures at HRC

Seed supplier abbreviations are: JSS = Johnny’s Selected Seed, HM = Harris Moran, RMSC = Rocky Mountain Seed Company, SOC = Seeds of Change, and V = Vilmorin.


The green leaf varieties were more uniform in early development, and showed the greatest difference in days to bolting.

Black-seeded Simpson, Concept, Crisp and Green, and Envy green leaf lettuce varieties
Grand Rapids, Green Vision, Marin, and Saladbowl green leaf lettuce varities
Black-seeded Simpson, Concept, Crisp & Green, Envy
Grand Rapids, Green Vision, Marin, Saladbowl
Simpson Elite, Two Star, and Waldmann's Dark Green green leaf lettuce varieties
Simpson Elite, Two Star, Waldmann’s Dark Green

Graph of Green of Leaf Lettuce Time to bolting by variety and sowing date

Bar Graph of Green Leaf Lettuces, Bolting by sowing date

Black Seeded Simpson (RMSC) – This old variety fails to hold up in Colorado ‘s summer weather. It bolts very quickly all summer long.

Concept (JSS) – Held relatively well, at least 75 days from seeding, all summer. Flea beetles hit hard in early July, thick sturdy erect leaves, heavy head, nicest late season. Second best for tip burn resistance.

Crisp and Green (JSS) – Held less well than Concept but slightly better than Envy. Safe for 75-80 days. Middle of the pack in terms of tip burn.

Envy (JSS) – Uniform, and reliable to 75 plus days. A frilly Waldman type, slightly lighter green in color. Less tip burn than other Waldmann types. Slightly less tip burn than Crisp and Green or Simpson Elite.

Grand Rapids (RMSC) – Similar holding and quality to Crisp and Green. Highest degree of tip burn in the green leaf lettuces trialed.

Green Vision (JSS) – Did not hold up quite as well as Crisp and Green and Grand Rapids , but is very similar. Not as much tip burn as Grand Rapids , but more than others.

Marin (JSS) – Holds reliably to 75 days, has relatively less tip burn than others in its class.

Salad Bowl (RMSC) – Holds very long in the early season, but bolts quickly in the heat. This variety is also attractive to aphids, and makes a good reservoir for beneficial insects.

Simpson Elite (RMSC) – Results were quite variable, but this variety held the longest directly after the high July temperatures. It is in the middle of the pack in terms of tip burn.

Thai Green (SOC) – Fair to poor performance overall, with a high degree of tip burn.

Two Star (JSS) – Similar holding capacity to Green Vision, this variety had the least tip burn.

Waldmann’s Drk. Grn . (JSS) – Variable results with this variety make conclusions difficult to draw, however only half of the plantings held past 70 days. Tip burn was the highest of all varieties.

Waldmann’s Green (RMSC) – Very similar results to Waldmann’s Dark green , but with bolting occurring less than 70 days in two-thirds of the plantings.


Flea beetles in early season left the red leaf lettuce alone. There was a high degree of disease in the early plantings however, which was associated with a leafhopper vectored phytoplasm.

Blackjack, Bronze Arrow, Galactic, New Red Fire lettuce
Blackjack, Bronze Arrow, Galactic, New Red Fire lettuce
Blackjack, Bronze Arrow, Galactic, New Red Fire
New Red Fire, Prizehead, Vulcan, Red Sails

Bar Graph of Red Leaf LEttuces, Days to Bolting by variety and sowing date

Bar Graph of Red Romaine Lettuces, Bolting by Date of Sowing

Blackjack (JSS) – Did not hold very well. Deepest red, nearly black leaves, slow starter, small, not vigorous, good for salad mixes

Bronze Arrow (SOC) -Held well. Red Oak leaf type, very attractive green to red mottled, excellent quality early, powdery mildew susceptible late

Galactic (JSS) – Held well but failed to size. Wine red, good for baby lettuce

New Red Fire (HM, JSS) – Held the best for the red Grand Rapids type. Good performance later in the summer, making good size heads, and holding well. Color is not as deep red as some.

Prizehead (RMSC) – Best performance is late summer, earlier plantings did not size or hold well. Tip burn was a problem during times of higher temperatures.

Rave (JSS) – Holds well, but sized very slowly. Very frilly, compact head, nice color and texture for salad mixes.

Red Sails (RMSC) – Common red leaf, good color, holds fairly well, but not as well as New Red Fire, and slightly better than Vulcan.

Red Salad Bowl (RMSC) – Holds nearly as well as Bronze Arrow, a similar cut leaf type.

Royal Red (RMSC) – Failed germination in all attempts

Vulcan (JSS) – Safe for early and late production, but didn’t hold well mid season.



The Romaine varieties, while generally slower developing than the leaf lettuces all held over 80 days before bolting. Head character was quite different between varieties. Leaf hopper phytoplasm appears to have been the cause for early season disease.


Claremont, Cosmo Savoy, Green Forest, and Green Towers Romaine lettuce varieties
Jericho, Medallion, and Paris Island Romaine lettuce varieties
Claremont, Cosmo Savoy, Green Forest, Green Towers
Jericho, Medallion, Paris Island

Bar graph of Romaine lettuces, Days to bolting by variety and sowing date

Claremont (JSS) – A “mini romaine”, held well in the field, nice quality, but is very small. Tip burn was common in mid season plantings.

Cosmo-Savoy (SOC) – Along with Paris Island consistently held to 90 days, but not much longer. Savoy leafed with light green color

Green Forest (JSS) – Held the longest of the full size, conventional type Romaines. However its performance was not as consistent as Green Towers. Flea beetles caused early damage which it grew out of.

Green Towers (HM) – Consistently held between 90-100 days, but was hurt by disease early in the season.

Jericho (SOC) – Quite variable in performance resulting in incomplete results. Cabbage color and texture most of the season, better quality late season. Leafhopper phytoplasm was significant.

Medallion (JSS) – Held over 90 days. Cabbage-like color and texture. Susceptible to the leafhopper phytoplasm.

Paris Island (RMSC) – Not the longest to bolt, but consistently in the 85-95 day range

Paris White (RMSC) – Removed from study due to poor germination


The red romaines that were grown failed to produce a dense romaine head, but rather had open habits with very large leaves on a stock that tends to stretch as if starting to bolt. These lettuces might be better suited to whole leaf processing, or grown as baby lettuces.

Rouge d'hiver, Cimmaron, Integrata, and Eruption red romaine lettuce varieties
Rouge d’hiver, Cimmaron, Integrata, Eruption

Bar Graph of Red Romaine Lettuces, Days to Bolting by variety and sowing date

Rouge d’hiver ( JSS ) – Held better than any of the others in this class, consistently over 80 days from seeding. Tip burn was a problem during the warmest weather.

Cimmaron ( HM, RMSC ) – Held about as well as Integrata and Eruption. Large oval leaves in a loose head habit, attractive green to red. Disease kept this variety from producing a uniform stand.

Integrata ( JSS ) – Fair to poor holding through the heat. Very attractive ruffled leaves.

Eruption ( JSS ) – Fair to poor holding through the heat, more frilly leaf than Integrata. Disease was a significant problem.


These lettuces stand out as the most resistant to bolting. All of the varieties held throughout the season, and were harvestable for many weeks. The crisp texture and good flavor are added bonuses. Flea beetle damage and tip burn were significant in the early plantings, but other disease and pest pressures were minimal.

Sierra, Nevada, Tahoe, and Rouge de Grenoblouse Batavian lettuce varieties   Nevada
  Rouge de Grenoblouse

Bar graph of Batavian lettuces, time to bolting by variety

Bar graph of Batavian Lettuces, bolting by sowing date

Nevada (V) – Green. Very long holding, none has bolted in the trials, good quality, thick and crisp leaves, flea beetle damage outer leaves

Rouge de Grenoblouse (SOC) – Red, savoyed leaves, tip burn is significant.

Sierra (V) – Very long holding, none has bolted in the trials, good quality, thick and crisp leaves, flea beetle damage outer leaves

Tahoe (V) – Very long holding, none has bolted in the trials, good quality, thick and crisp leaves, flea beetle damage outer leaves


These delicious lettuces hold reasonably well, and are harvestable earlier than the other types. Tip burn and leafhopper phytoplasm caused significant problems.

Lochness, Winter Density, Optima
Lochness, Winter Density, Optima, Buttercrunch, and Dark Green Boston butter lettuce varieties
Buttercrunch, Dark Green Boston

Bar Graph of Butter Lettuces; Time to bolting by variety and sowing date

Bar Graph of Butter Lettuces; Bolting by Sowing Date

Buttercrunch (RMSC, SOC) – Held well over 90 days, but was susceptible to disease at the highest rate of all the butters, and had slightly higher than average tip burn.

Dark Green Boston (RMSC) – Held between 70 and 85 days. Tip burn was slightly worse than the other varieties.

Divina (V) – Too few surviving plants to evaluate. Emergence and survival in greenhouse was poor.

Optima (V) – Holding over 90 days for all but one planting, this variety made very large heads, and was mid range for disease susceptibility and tip burn.

Lochness (V) – Held over 90 days, made nice large heads and was slightly less affected by disease and tip burn than Optima.

Winter Density (SOC, Crisp Butter) – This lettuce combines the form of a mini romaine and a butter lettuce, creating a very dense, smooth textured head. Nice quality late summer, but was quite susceptible to disease. Damage by tip burn was the least of the butters.


Sierra variety - Batavian lettuce
The Batavian lettuce varieties outperformed the other varieties of lettuce by far. They did not bolt all season long, even in the warmest days. They are flavorful and have uniform growth. They are longer maturing than the leaf lettuces, but they can be harvested at different stages. Overall they are a versatile and good performing lettuce, and are highly recommended for Colorado growers.

The Butter varieties also performed well. The Red Romaines failed to produce a head but are good for individual leaf harvest and baby lettuce production.

Results of the antioxidant studies are not yet available.


This research was expanded during the summer of 2004 to 100 varieties. The antioxidant research was also continued and taste tests were incorporated for a few varieties. Results will be posted at a later date.

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