Using Grafting Technology to Enhance Greenhouse Cucumber Production in High Tunnels

What we found during background information collection

• Not all the farmers understand technical words commonly used to describe cucumber varieties. 
• In terms of seedless cucumber varieties, Corinto is a popular slicing cucumber, but growers are also interested in other types of cucumbers, particularly long-type cucumbers. 
• In USDA hardiness zone 5 and 6, cucumbers were planted in end March to early May in unheated high tunnels. If heating is available, growers would target transplant as early as end Feb.

2017 results

The difference in the transplant survival rate of grafted vs. non-grafted cucumber plants was observed in the open-field trial at the Bud’s farmers market. 52% of non-grafted Diva cucumber seedlings died 7 d after transplanting, while all the grafted Diva plants survived. Following transplanting, the lowest recorded air temperature on the site was 46 °F. Average air temperatures in the week following transplanting ranged from 52-73 °F. The observation indicated that grafted cucumbers are more likely to survive during suboptimal temperature conditions that often occur in the early spring.

Pronounced yield increase of grafted cucumbers was observed at White Violet Center. The highest yield increase was observed on Excelsior (159%). One of the reasons that lead to the dramatic yield difference was because of plant loss due to bacterial wilt. Almost all the non-grafted Excelsior plants were died becuase of bacterial wilt while more than half of grafted Excelsior plants were still producing in July (Fig. 1). Although the different response toward bacterial wilt of grafted vs. non-grafted cucumber plants was observed at the White Violet Center, no difference on plant survival under bacterial wilt pressure was observed at Freedom Valley Farm, at where, regardless of grafted or non-grafted plants, a tremendous amount of plants were lost due to bacterial wilt in June. Yield increase of other cucumber varieties ranged from 27% to 97% at the White Violet Center. Grafted plants had stronger stems compared to non-grafted cucumbers (Fig. 2) The farmer is very happy with the performance of grafted cucumbers.

Grafted Corinto increased yield by 6% at Freedom Valley Farm. As Corinto and Sweet Success seem to be more cold tolerant compared to other greenhouse cucumbers. Response of the two varieties to grafting in terms of the improved cold tolerance may be decreased. However, according to growers’ observation, grafted plants were taller and more vigorous compared with the non-grafted plants.

Two trials were conducted at Bud’s Farmers Market, one in a high tunnel and another one in the open-field. More than half of transplant failure of non-grafted Diva lead to a significantly lower yield of non-grafted plants compared to grafted Diva. The farmer also indicated that he normally planted cucumbers every 4-6 weeks. But growing grafted plants allow him harvesting cucumbers from June to Sep. with one planting, which dramatically saved his labor to grow transplants and field planting. 

Insects netting was installed in the high tunnel at Bud's Farmers Market that effectively controlled cucumber beetles. No bacterial wilt was observed. Unexpected, we saw a negative effect of grafting in this trial. Yields of grafted plants were lower than non-grafted plants with all the evaluated varieties. We suspect it is the poor quality of grafted plants that lead to the lower yields. It is possible that the grafted plants were not fully recovered before planted in the high tunnel at Bud’s Farmers Market. The trial will be repeated at the same site in 2018, which could provide us more opportunities to better understand why grafting did not work in this case.

Table 1: Cucumber yield at the Freedom Valley Farm (Harvest duration: May 5 --- June 17)

Table 2. Cucumber yield at the White Violet Center (Harvest duration: May 12-July 19)

Table 3. Cucumber yield of high tunnel trial at the Bud's farmers market (Harvest duration: May 1-June 13)

Table 4. Cucumber yield of open-field trial at the Bud's farmers market (Harvest duration: June 5-Sep 15)

2018 results

Ten on-farm trials were conducted in 2018. We will discuss the results of two major production systems. 

Heated greenhouses

A pronounced advantage of using grafted cucumbers was observed in the situations that cucumbers were grown in soils in heated greenhouses. 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 John Raber's farm, the non-grafted cucumbers struggled badly and never seemed to put out new growth (Figure 3). The grower eventually decided to take out all the non-grafted plants and replanted cucumbers later in the season.

At the Weathered Plow farm, a pest damaged cucumber stem right above soil-line that caused about 30% plant loss at the end of March (Figure 4). 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 (Table 5).

Table 5. Yield per plant of non-grafted Corinto and Corinto grafted onto Titan rootstock at the Weathered Plow farm. 

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. Jim Baughman at Freedom Valley Farm 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. He 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, Jim 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 the Perkins’ good earth 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.

Table 6. Yield per plant of non-grafted and grafted Iznik and Socrates at the Perkins Good Earth Farm. 

Table 7. Yield per plant of grafted and non-grafted Corinto at S&E Produce & Flower farm. 

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 at Bud's Farmers Market. The same trend was observed in both 2017 and 2018 at the same high tunnel. A unique situation for this greenhouse was very high, above 8 soil pH and relatively higher soil salinity. We suspect the grafted plants may be more susceptible to high soil pH and salinity. Further research in this area is needed. 

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 (Figure 4), 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.

Economic analyses 

We estimated the cost of grafted cucumber plants is about $1.36 per plant, which is 2.3 times more expensive than the normal cucumber plants. 

Table 8. The estimated cost of grafted vs. normal cucumber plants 

Then we conducted partial return analyses based on yield results from research trials conducted from 2016-2018. The analyses indicated that assuming all the production costs, expect transplant cost, are the same for growing grafted and non-grafted cucumbers. Growers can make more money in growing grafted plants (Figure 5). This analyses used a flat cucumber price $1.5/lb and did not consider potential premium price in the early season. We are conducting more in-depth studies to evaluate the economic feasibility of using grafting technology in high tunnel cucumber study.