Effects of increased humidity on lettuce yield on three small-scale, arid vegetable farms

Commodities

• Vegetables: greens (lettuces)

Practices

• Crop Production: irrigation, low tunnels, row covers (for season extension), season extension, water management, other

Arid, high desert vegetable growing is marked by large diurnal and seasonal temperature swings, low humidity, and high risk. It is a tricky business infrequently written about by farmer-authors or studied in academic research. This proposal is a collaboration of three arid vegetable growers to research better growing techniques in our climate, provide educational opportunities to fellow growers, and add to the body of knowledge.

Lettuce is one of the most important crops for small-scale vegetable farms, but extremely challenging in the summer. Most SARE projects and extension publications investigating summer lettuce production test variety heat-tolerance or temperature effects of shade cloth. While variety choice is important, the impact of shade on yield varies in these studies.

While humidity is popularly understood as a problem, field observations under remay and shaded plastic high tunnels which trap moisture have made us question that received wisdom for our arid conditions. This leads to our research question: in arid climates, does increasing relative humidity improve summer lettuce yield in June, July, and August?

We propose to test 30 percent remay low hoops with drip or overhead micro-irritation to increase humidity and yield in lettuce beds. We will compare lettuce yield of a common variety to relative humidity across three farms and two summer growing seasons.

We will share research results, but also strategies for improved germination, transplant survival, and yield in arid growing. We will help growers understand the impact of temperature, humidity, and soil water tension on plant growth, and the importance of monitoring and low-tech, sustainable ways to manipulate conditions.  

This research would be useful to growers in dry two-thirds of our state and throughout the west. What we learn in the high desert may become more relevant for successful vegetable growing in other regions due to climate change.

Research Objectives:

• Objective 1: Measure differences over two growing seasons in lettuce yield per bed foot between uncovered beds irrigated with drip vs overhead micro-irrigation. 
• Objective 2: Measure differences over two growing seasons in lettuce yield per bed foot between covered beds (low hoops of 30% shade remay) irrigated with drip vs overhead micro-irrigation.
• Objective 3: Monitor relative humidity (RH) two seasons and analyze correlation between RH and lettuce yield. 
• Objective 4: Monitor ambient temperature and bed temperature and soil water tension and analyze relationships to RH under study treatments.

Education Objectives:

• Objective 1: Discover latest research on vegetable production in the arid western US through five interviews with researchers on vegetable production in the arid west by December 2025. 
• Objective 2: Discover common challenges for vegetable growers in the arid western US through five interviews with growers by December 2025. 
• Objective 3: Disseminate findings to arid vegetable growers through the following activities by December 2026:
  • A series of publications with Oregon State University (OSU) Extension and Growing for Market magazine. GFM has 5,000+ subscriptions.
  • Two podcast episodes with Growing for Market. GFM has about 200,000 downloads per episode.
  • An in-person workshop on our SARE research and arid vegetable production at the OSU Small Farms conference. The conference typically draws 1000 producers and agricultural service providers with 50-100 participants per workshop. 
  • A series of three online workshops with OSU on arid vegetable growing in year 2 focused on 1. strategies to improve summer germination, transplant survival, and yield, 2. the importance of humidity, temperature, and soil water tension in arid growing, and 3. how to monitor these factors on the farm for better decision making. We expect approximately 100 producers to attend.