OSU Kabocha/Buttercup Cultivar Production and Storage Project: 2015 Results

Authors: Alex Stone and Jenny Wetzel, OSU Horticulture


The goal of this project is to identify high-yielding, long storing and delicious kabocha and buttercup cultivars for production on WIllamette Valley fresh market vegetable farms. Seventeen cultivars of kabocha and buttercup winter squash were grown at four planting densities in a replicated field experiment at the Oregon State University research farm in 2015. Data was collected on fruit size, yield, storage quality, flavor, resistance to storage rots and soil borne disease, and duration of storage.  This was our first year evaluating these cultivars; we will repeat this experiment in 2016. The OSU experiment was managed conventionally as there is insufficient organic land at the Corvallis research farms. Single-density replicated on-farm trials were also conducted on two organic farms. This article primarily reports on the methods and results from the OSU research station trial.


Cultivars were selected based on grower recommendations, availablity as organic seed, and performance in other regions (Table 1).

Table 1.  Squash Cultivars

Design and Management
Each cultivar was grown at 4 densities with 4 replications in a replicated complete block design. S
quash transplants were grown in the greenhouse and transplanted into the field in late May. Rows were planted on 5 ft centers; density varied by in-row spacing (8, 12, 20 and 30 inches between single plants). 

Data Collection
Fruits were harvested the first week of September. All fruits from each plot were weighed and counted at harvest. Fruit were stored in a walk-in cooler with dehumidifer that was maintained at 55o F and 60% relative humidity. Fruit were evaluated periodically from September 2015 through March 2016 for percent rotten or otherwise unmarketable (ie. soft/wrinkled), and a subsample was evaluated for external and internal color, percent Brix, and dry matter. On several dates some cultivars were evaluated by a chef for sensory quality (data not yet available).


Highest Performing Cultivars in 2015

          Green Kabochas

Bagheera     Black Forest        Cha-Cha          Delica             Sweet Mama         Thunder

The stand-out in this category was the 1979 All America Selection Sweet Mama (http://all-americaselections.org/winners/details.cfm?WinID=487). Sweet Mama yielded approximately 32 tons/acre when planted at the 12 inch in-row spacing.  Sweet Mama is large-fruited, very long storing and resistant to storage rots, and can be sold from October through January. Sweet Mama has a semi-bush habit so it sets all of its fruits early and there are very few immature fruit in the field at harvest. Sweet Mama has excellent sensory and culinary quality.

          Gray Kabochas

   Blue Kuri            Winter Sweet           Confection

The stand-out in this category was Winter Sweet.  Winter Sweet yielded approximately 25 tons/acre when planted at the 12 inch in-row spacing.  Winter Sweet is extremely resistant to storage rots and very long storing. Winter Sweet is not edible at harvest as its dry matter is too high and its sugars too low.  It must be ripened for several months before marketing and should be sold from December through March.  The skin color lightens from greenish-gray to light gray and then blushes pink as the fruit ripens.  Winter Sweet is also delicious and remains so into March.

          Red Kabochas

    Sunshine               Eastern Rise             

Sunshine, a 2004 All America Selection (http://all-americaselections.org/winners/details.cfm?WinID=414) outperformed Eastern Rise in our 2015 trials.  Sunshine yielded approximately 27 tons/acre when planted at the 8 inch in-row spacing. It has an attractive deep orange skin and orange flesh color that do not change during storage.  Sunshine can be marketed from October to December.  Neither of the red kabocha varieities were resistant to storage rots in our trials. Eastern Rise is reported to outperform Sunshine when grown in cool summers (Fedco catalog; http://www.fedcoseeds.com/seeds/search?item=1633).          


     Bonbon                 Burgess                     Discus         Uncle David's Dakota Dessert

The stand-out buttercup was the 2005 All America Selection Bonbon (http://all-americaselections.org/winners/details.cfm?WinID=4210).  Bonbon is a high yielding large buttercup with an attractive glossy skin and a round shape.  It yielded approximately 27 tons/acre when planted at the 12 inch in-row spacing.  Bonbon is a semi-bush variety that leaves few immature fruit in the field.  None of the buttercup varieties were resistant to storage rots in our trials.  Bonbon can be marketed from October through December. Bonbon has very good sensory and culinary quality.



Tetsukabuto was the highest-yielding and longest-storing cultivar in the entire trial with an average yield of 31 tons/acre (across all densities).  It yielded approximately 39 tons/acre when planted at the 12 inch in-row spacing.  Tetsukabuto is a maxima moschata cross that is most commonly grown as a rootstock for melons due to its resistance to soilborne diseases.  Tetsukabuto was the only variety in our trials that exhibited resistance to the soilborne disease problem that is reducing winter squash yields in the valley.  Tetsukabuto is also extremely resistant to storage rots, very long-storing, and resilient to fluctuating storage conditions (e.g. a barn).  Like Winter Sweet, it is not edible at harvest; it must be stored for several months before eating/marketing and should be marketed from December through March.  The fruits at harvest are uniformly dark green.  As the fruit ripen they develop yellowish green stripes that turn to orange; later in storage the fruits become uniformly tan.  Tetsukabuto does not produce viable pollen and must be planted with a pollenizer (other maxima or moschata variety).  Tetsukabuto is also very good eating.


The highest yielding cultivars were Tetsukabuto, Sweet Mama, Sunshine, Bonbon and Winter Sweet.  Yield potential of the cultivars is shown in the figure below. Yields are shown for the planting density (in parentheses) at which that cultivar was numerically highest-yielding.  Fruit number and fruit weight contribute to yield. As density (plant number) increases, fruit number increases and fruit weight decreases, and these dynamics vary by variety.  For this reason, optimal planting density is not the same for all varieties.

Fruit weight/size

An important attribute of a winter squash cultivar is fruit weight or size.  Average fruit weight (across all densities) of each variety is shown in the figure below. 


Fruit size as related to planting density

Fruit size typically declines as planting density increases, as shown in the figure below. The exception in our trials was Tetsukabuto; its fruit size did not change significantly with density.


Fruit per plant as related to planting density

An important contributor to yield is the number of fruit per plant. For cultivars that do not set very many fruit per plant (true for many kabochas), increasing planting density is the best strategy to increase yield per acre. As planting density increases, the number of fruit per plant typically decreases, as shown in the figure below. Yield can increase, despite the lower number of fruit per plant, as the number of plants increases.


Susceptibility to storage rots 

Some cultivars are more resistant to storage rots than others. In OSU storage trials, Tetsukabuto was the most resistant, followed by Winter Sweet and Sweet Mama. All other cultivars, including Sunshine, were susceptible to storage rots. In squash grown in an on-farm trial and stored first at the farm and then in the OSU walk-in cooler, Sunshine also appeared to be resistant to storage rots (see figure below). The take-home message from the OSU and on-farm data is to sell the more susceptible varieties in the fall and save Sweet Mama, Tetsukabuto, and Winter Sweet for winter sales.


Ripening and marketing windows

Cultivars become ripe and then over-ripe over time and the timing of ripening differs amongst cultivars. We measured the percent Brix and dry matter of a subsample of fruit for each cultivar throughout the fall, winter and spring.  We are still analyzing the entire data set.  The data for Sweet Mama and Winter Sweet are shown below. The blue area represents a possible marketing window for these cultivars based on the ripening and storage rot data. Susceptiblity to storage rots is another important determinant of marketing, as squash that are susceptible to storage rots must be sold before they rot.

While there is not very much information on the relationship between dry matter and percent Brix and eating quality, it likely that for kabocha and buttercup types dry matter should be between 15 and 20 percent and Brix should be at or above 9 or 10.  In our trials, Sweet Mama average dry matter was 22% at harvest, while Winter Sweet dry matter was 30%, which is so high that it was not edible (it tastes like cement). Sweet Mama was of this quality starting in October after a few weeks of ripening to reduce dry matter and increase Brix. Winter Sweet required two months of ripening to reduce dry matter to 20% and increase Brix to 10 before it was of good eating quality.

Sweet Mama dry matter declined to about 10% and percent Brix to 8% in January (and at this same time it started to rot). In contrast, Winter Sweet dry matter remained at or above 15% and percent Brix was at or above 10 (and it did not start to rot). Based on all of this information, Sweet Mama stored under these conditions was losing quality by early January, while Winter Sweet was maintaining quality and was still marketable in February. 




The table below reflects our current hypotheses on when all of the cultivars we evaluated should be sold based on all of these variables and under the OSU storage conditions.  Most can be marketed from October (several weeks of ripening after harvest) through December or possibly into January. Sweet Mama is both resistant to storage rots and long storing, so it has the potential for a longer marketing window.  Winter Sweet and Tetsukabuto need several months of ripening before achieving good eating quality, but then maintain that quality into the middle of winter; they should be marketed from December through February or March. This table is based on our experiences with these cultivars in one location and from a single year of data collection; farmers must vet this information for their locations and storage environments and over several production seasons.

In the table below, dark blue shading represents the primary marketing window for that cultivar. The lighter blue shading represents the potential for an extended marketing window when squash is storing well.

Table 2.  Possible Marketing Windows (based on 2015-16 data)