In-Field Forcing of Rhubarb

Rhubarb growers would benefit from an inexpensive, reliable method for forcing crowns into early production and increasing yields. Early harvests of high quality spears allow growers to take advantage of the usually higher early season prices. Currently, two methods are commonly used to bring rhubarb into early production. In the first, crowns are dug, removed to a hot house, and forced under etiolated conditions. In the second, rows of crowns are covered in the field with clear plastic mulch which increases air and soil temperatures and results in early spear growth. The first method has the disadvantage that the crowns are exhausted and the field must be replanted. In the second method there are the additional costs of plastic purchase and removal and possible difficulty in laying the plastic during unfavorable weather conditions.

Gibberellic acid (GA) has been used to stimulate growth of hot house-forced rhubarb, but has not been used successfully in the field. Lack of response in the field may have been caused by non-etiolated conditions or poor uptake of GA by crowns or buds. Successful GA treatment of rhubarb may depend upon soil- and debris-free buds or may require injection.

The following experiments were designed to determine whether yield and earliness of field grown rhubarb could be increased by GA injection directly into crowns, including single and multiple injection points, or by a spray treatment of previously washed crowns.


In 1981, treatments were applied to crowns in a commercial planting on February 20, as buds were just beginning to break. A total of 10 ml of each treatment solution was injected into three separate buds (approximately 3-4 ml/bud) per crown. Plots consisted of four crowns each and treatments were replicated 10 times. Treatments were: 1) 10% ethanol in water (control), 2) 800 ppm GA in 10% ethanol, and 3) 4,000 ppm GA in 10% ethanol. One day following application of treatments, five replicates were covered with clear plastic mulch. Non-covered plots were harvested on 3/31/81.

In 1982, the treatments were applied to the North Willamette Experiment Station rhubarb variety planting. Treatments included 1) water-injected controls, 2) 10 ml/crown of a 500 ppm GA3 solution injected at three sites (approximately 3 ml each) per crown, 3) 10 ml/crown of 2,000 ppm GA3 at three sites, 4) 10 ml/crown of 2,000 ppm GA3 at one site per crown, 5) water-injected and covered with 2 mil clear polyethylene mulch, 6) poly mulch plus 2,000 ppm GA3 at three sites, 7) 2,000 ppm GA, 10 ml, three sites, injected in late spring, rather than winter, to once-harvested crowns, 8) washed crowns, sprayed with 100 ml of 50 ppm GA3, 9) washed crowns sprayed with 100 ml of 500 ppm GA3. All treatments were applied on February 12 except 7), applied on June 11.

Each type of treatment (rates of injection, single vs. multiple injection, sprays, and late applications) had its own set of controls. All experiments were in randomized block design with two crowns/plot and three to six replications of each treatment. Plastic-covered plots were harvested on March 18; GA application rate, and single vs. multiple injection treatments were harvested on April 8, spray treatments on April 23, and the late injection plots were harveste on July 25, 1982. No fertilizer or herbicide treatments were made in 1982 before the final harvest. Plots were not irrigated during the period of the experiment.

Results - 1981

GA markedly increased the total weight of rhubarb cut in a once-over harvest in 1981 (Table 1). The lower rate significantly increased yield and the higher rate further increased total yield but not marketable yield. Marketable spears were defined as at least 20 cm length and free of serious blemishes. Gibberellin treatment had very little effect on the number of spears harvested (Table 1), but significantly increased average spear weight and length (Table 1). Response to GA was greatest within 5 to 7 cm of the injection site. Untreated areas of the crown showed little response, indicating very little translocation of the GA. Quality of the spears from treated crowns was adequate, comparing favorably with quality of plastic-forced spears. There was some scarring of spears originating at the injection point, apparently caused by mechanical damage to the bud.

The plastic-covered plots were not harvested but GA treatment effects on spear length were evident. However, the degree of GA response under plastic appeared smaller because of increased growth of spears on control crowns.

Table 1. Effect of GA on Rhubarb Yield and Spear Size, 1981                               Total    Marketable    #Spears/    Mean    Mean spear  Treatment         yield      yield        crown	     spear     length                  (kg/plot)  (kg/plot)                 wt.(g)     (cm)    Control            2.73      0.00          103         19       10.7  GA, 800ppm         5.20      1.60          134	       32       21.6   GA, 4,000ppm       7.27      1.95          150         36       24.1       LSD (0.05)    1.83      0.46           NS          6        4.2    

Results - 1982

Rate of Injection
As in 1981, injection of GA3 at three sites per crown increased total yield at once-over harvest on April 8, 1982, increased the number of spears/crown, increased mean spear weight, and increased the number of spears exceeding 20 cm in length (Tables 2 to 5). However, very little increase occurred with the 500 ppm solution, particularly for the 'Crimson' variety. For each of the above parameters, there were statistically significant increases only with the 2,000 ppm solution. In 1981, both 800 and 4,000 ppm solutions caused significant increases in yield. Apparently, injection of at least an 800 ppm solution or 8 mg GA/crown is necessary for an effective response. Environmental conditions may also affect the dose response, however.

Combination with Plastic Mulch
GA3 injection (2,000 ppm) also increased yield, mean spear weight, and number of spears more than 20 cm long in the presence of clear plastic mulch (Table 6). The experiment was replicated over two clones of 'Victoria'. No significant clonal differences occurred and data presented are means for the clones. After plastic removal and clean harvest, regrowth was more rapid on GA-treated crowns. However, the yield difference at second harvest was not significant (data not shown).

Single vs. Multiple Injection
The effect of multiple vs. single injection appeared very striking within the first three weeks following injection of 'Victoria'. Accelerated growth on singly injected crowns occurred only near the injected bud; the multiply injected crowns usually had three distinct sites of rapid growth. However, there were no statistically significant increases in yield components for multiply vs. singly injected crowns. Apparently, any differences that might have occurred were obscured by normal variation, or the injected GA may have slowly diffused to other growing points (yield data not shown).

Spray Treatments
Some early stimulation of growth by spray application of GA was evident but early differences were not reflected in any yield component (data not shown). Direct injection may be necessary to insure the GA growth response. The high rate spray treatment resulted in application of 50 mg GA/crown compared to 20 mg GA/crown for the high rate injection treatment. Much of the sprayed material was lost as runoff and absorption through the buds was small.

Late Spring Injections
Application of GA in June after harvest of the first crop had no statistically significant effect on yield and there was no stimulation of early regrowth (data not shown).

Table 2. Effect of Gibberellin Injection on Total Yield of Rhubarb, 1982           GA treatment  Variety    Total spear         Mean for GA         Mean for variety                          wt. (kg/crown)   treatment (kg/crown)        (kg/crown)    Control       Crimson       4.00         Control     2.86        Crimson     4.26                 German Wine   1.58         500 ppm     3.38        German Wine 2.53                 Victoria      2.99         2,000 ppm   4.90        Victoria    4.35                ------------------          LSD (0.05) 0.63  500 ppm       Crimson       3.79	                    German Wine   2.17                 Victoria      4.19                -----------------  2,000 ppm     Crimson       5.00                 German Wine   3.84                 Victoria      5.87                                                       Table 3. Effect of Gibberellin Injection on the Number of Spears    Harvested per Crown, 1982                                                   	  GA treatment   Variety    Total spears/    Mean for GA      Mean for                                crown         treatment        variety          Control        Crimson         70          Control    82    Crimson      74                  German Wine     67          500 ppm    91    German Wine  90                    Victoria       110          2,000 ppm 117    Victoria    127                 ------------------          LSD (0.05) 11  500 ppm        Crimson         69	                    German Wine     80                  Victoria       124                 ------------------  2,000 ppm      Crimson         82                  German Wine    124                   Victoria       146                                                 Table 4. Effect of Gibberellin Injection on Mean Spear Weight, 1982           GA treatment   Variety   Mean spear     Mean for GA	        Mean for                             wt. (g)      treatments(g)          variety (g)    Control        Crimson       57         Control      36     Crimson       58                  German Wine   24         500 ppm      39     German Wine   27                  Victoria      27         2,000 ppm    44     Victoria      34                 -----------------          LSD (0.05)  6  500 ppm        Crimson       55	                     German Wine   27                 Victoria	     34                 -----------------  2,000 ppm      Crimson       61                  German Wine   31  	                 Victoria      40                                                   Table 5. Effect of Gibberellin Injection on Numbers of Spears/Crown  	  Exceeding 20 cm in Length, 1982                                               GA treatment   Variety   Spears/crown    Mean for GA             Mean for                            over 20cm       treatment              variety      Control        Crimson	     28          Control     23     Crimson       32                 German Wine   10          500 ppm     35     German Wine   31                 Victoria      30          2,000 ppm   57     Victoria	  52                 ----------------           LSD (0.05) 26  500 ppm        Crimson	     29                                 German Wine   29                  Victoria	     47                 ----------------  2,000 ppm      Crimson       38                  German Wine   54  	                 Victoria      78                                                 	  Table 6. Effect of Gibberellin Injection on Yield Parameters of  	  Plastic Mulch Covered 'Victoria' Rhubarb, 1982                                     GA treatment    Total spear     Number of spears   Mean spear   Number of spears/                 wt. (kg/crown)   harvested/crown	   weight (g)   crown over 20 cm  Control            2.44               84               28             11   2,000 ppm          4.01               85               49             36         LSD(0.05)    1.07               NS               11             13