Performance of Heat-tolerant Lettuce Cultivars

Lettuce (Lactuca sativa) is a high-value crop cultivated worldwide. Harvested lettuce acreage in New Mexico, USA, trails the leading lettuce production states (California, Arizona), but growers in New Mexico are interested in expanding their production. To do this, farmers need consistent yields and delayed bolting to support scaling acreage and meeting market windows with a dependable supply.

To help New Mexico farmers reach new markets, locally relevant information on heat-tolerant cultivar performance is needed because heat and early bolting can limit yield. Researchers at NMSU tested “heat-tolerant” varieties to identify those best suited to southern New Mexico. The study was conducted in 2020–2021 at the NMSU Heritage Farm in Las Cruces and evaluated six lettuce cultivars described as heat tolerant by seed suppliers or other sources.

Here’s a quick summary of the trial:

• Trialed 6 lettuce cultivars marketed as heat tolerant:

  • Butterhead: ‘Anuenue’, ‘Mikola RG10’

  • Green leaf: ‘Muir’, ‘Tropicana’

  • Romaine: ‘Parris Island Cos’, ‘Sparx’

• Measured:

  • Marketable harvest weight (yield)

  • Days from transplant to first bolt

  • Days to 50% bolting

Key results for farmers:

• Romaine types yielded the most.

  • In 2020, ‘Sparx’ averaged 32% higher yield than the other lettuce types.

  • In 2021, ‘Sparx’ and ‘Parris Island Cos’ produced about 19% more marketable yield than the other cultivars.

• Bolting resistance varied by year.

  • In 2020, ‘Sparx’ was the last to bolt (best for holding quality longer in heat).

  • In 2021, ‘Mikola RG10’ and ‘Muir’ were the slowest to bolt, suggesting value for extending the harvest window.

For southern New Mexico conditions, romaine—especially ‘Sparx’—looks like a strong option for higher marketable yields, while ‘Mikola RG10’ (butterhead) and ‘Muir’ (green leaf) may help growers push harvest later by bolting more slowly (at least in 2021).

Key benefits for farmers:

• Higher marketable yield in heat: The romaine cultivars—especially ‘Sparx’—produced more saleable lettuce under southern NM conditions (+32% in 2020; romaines +19% in 2021 vs. other types).

• Less loss from early bolting: Some cultivars held longer before bolting, helping protect head/leaf quality and reducing unmarketable product.

• Longer harvest window (more flexibility): ‘Mikola RG10’ and ‘Muir’ bolted later in 2021, suggesting they can help stretch harvest timing when heat pushes crops fast.

• Lower production risk when temperatures spike: Using proven heat performers can make warm-season plantings more reliable year to year.

• Better cultivar selection for NM: Provides locally tested information so growers can choose varieties with confidence instead of relying only on seed-catalog claims.

  For more information on growing zones and varietal recommendations check out the following publication https://pubs.nmsu.edu/_circulars/CR457B/

  Courtesy of NMSU