Cucumbers are extremely sensitive to cold. Locally grown cucumbers are almost only available in the summer. While in Asia, without the use of fancy heated greenhouses, cucumbers can grow all winter. Growing grafted cucumbers with cold tolerant squash rootstock is one of the key factors making this possible.
Since 2016, we started to evaluate opportunities of using grafted plants to extend early season cucumber production under protected cultural systems in the Midwest. We observed promising results in our research trials. However, knowing research trials can only tell part of the story, we initiated multiple on-farm trials across Indiana to better understand if and under what circumstances growers would benefit from this technique. This article discusses the lessons we have learned so far and raises questions that need to be answered.
Heated greenhouses
A pronounced advantage of using grafted cucumbers was observed in the situations that cucumbers were grown in soils in heated greenhouses (Figure 1). Trials were conducted in three greenhouses across Indiana. Cucumbers were planted in soils at the same time as early tomatoes were planted at the end of February. Soil temperatures across the three sites were in the lower 60°F in March, and 2 to 3 degrees higher in April. All the cucumber plants survived transplanting. At one location, the non-grafted cucumbers struggled badly and never seemed to put out new growth. The grower eventually decided to take out the non-grafted plants and replanted cucumbers later in the season. In another situation, an unknown pest damaged cucumber stem right above soil-line that caused about 30% plant loss at the end of March. Interestingly, this damage was only observed on non-grafted cucumbers. The grower also noticed that non-grafted cucumber plants had premature blooms, considerably earlier than the grafted plants. The observation indicated that the non-grafted plants were suffering from environmental stresses. Harvesting started in early April, mainly from grafted plants. Yields (per plant basis) from the remaining non-grafted plants were 65% and 39% lower compared to the yields of grafted ones in April and May. The growers commented that they have never harvested cucumbers that early, and they were very happy about the yield of the grafted plants. Although this is a favorable situation, some growers may face the challenge of finding buyers in April as most farmers’ markets in Indiana do not open until May. Another problem expressed by the grower is that they are uncertain about how to price the cucumbers in the early months.
Unheated high tunnels
Another significant benefit we observed on grafted cucumbers is a higher transplant survival rate under lethal soil temperatures in the spring in unheated high tunnels. Newly planted cucumber seedlings may wilt and die when average soil temperatures were around 55°F for two or more nights depending on the variety, the status of the seedlings, and temperatures during the day. A significant amount of plants will be lost if average soil temperatures are in the lower 50°F range. An advantage of using grafted cucumbers is that they are likely to survive in these lethal soil temperatures. A grower located in central Indiana commented that he is really amazed by the hardiness of the grafted plants. He lost more than 70% of non-grafted cucumber plants but none of the grafted ones after transplanted on March 31, 2018, in an unheated high tunnel. The grower reported a low air temperature inside the high tunnel at 21°F one morning in early April 2018. Data loggers recorded minimal air (under row covers) and soil (4’’-depth) temperatures were 33°F and 48°F in his high tunnel after transplanting.
Undoubtedly, the grower is very experienced at managing temperatures in an unheated high tunnel in the spring. But still, there is an extremely high risk of losing all the plants when temperatures dropped to that level. Actually, this is exactly what happened at another farm. The temperature around the plants dropped to 28°F, that temperature killed all the cucumber plants regardless of whether grafted or not.
Using grafted plants provides a higher chance for seedling survival if frost occurs after transplanting. This is surely an advantage of using grafted plants. However, we realize that in reality, growers may not take the risk of planting cucumbers in an unheated high tunnel if frost is expected shortly after transplanting. Most growers may wait until the frost has passed. So that they may not benefit from a higher transplant survival rate of grafted plants under lethal soil temperatures.
The next question is whether grafted plants bring higher yield if they were planted after the spring frost passed. Not surprisingly, we found this is very weather-depended. In 2018, we experienced an unusually cold April. Most growers delayed planting cucumbers in unheated high tunnels till about end April. May is quite warm in 2018. Average soil temperatures were in the upper 60°F in May and stayed above 70°F in June and July in our trials across Indiana. We saw little difference in plant growth and yield between grafted and non-grafted cucumbers. While in 2017, cucumbers were planted on March 31 and successfully established in two growers’ unheated high tunnels. Growers reported more vigorous and healthier looking grafted plants compared to non-grafted. Yield improvement ranged from 6% to 159% depending on varieties. In the situations that yield on grafted plants was almost doubled compared to non-grafted cucumbers, this was due to plant loss caused by bacterial wilt. However, we did not see differences in susceptibility to bacterial wilt between grafted and non-grafted cucumbers on other farms.
More questions need to be answered
A big surprise to us is that we noticed grafted plants were less healthy, and yield was reduced on grafted plants in one greenhouse. The same trend was observed for two years on the same farm. A unique situation for this greenhouse was very high, above 8 soil pH. It will be very interesting to find if the high soil pH contributed to the lower yield of the grafted cucumbers. Or if there are other factors involved.
The observation that unexpected pest damaged stems of non-grafted cucumber plants but not the grafted plants in one greenhouse was a surprise. Unfortunately, we were not be able to catch the pest and confirm it, but according to our entomologist, the damage looked like it was caused by seedcorn maggot, which is a common pest that occurs in early spring when the soil is cool. We would like to confirm this observation and understand why this happens.
Figure 1. Cucumbers were grown in a greenhouse in April 2018
This research is supported by the National Institute of Food and Agriculture, USDA through the North Central Region SARE program under project number LNC17- 390 and ONC17-027.