Prices paid for rhubarb are usually highest at the beginning of the production season. Growers would benefit from an inexpensive and reliable method for forcing crowns into early production. Hot house forcing has been used but this practice is expensive and the field must be replanted. Clear plastic mulch has been used to force rhubarb in the field but mulch costs may exceed $200/acre and plastic removal and disposal are an additional expense. Gibberellin plant hormone (GA) has been used to stimulate growth of hot house-forced rhubarb and has been used in the field on an experimental basis since 1980. These experiments have demonstrated that direct injection of GA into buds or crowns stimulates early spear production. Injection was shown to be more effective than spray treatments.
The objectives of the 1983 field experiments were 1) to evaluate the effectiveness of applying GA through a drip irrigation system, 2) to determine the optimum timing of GA application relative to calendar date or chilling degree day accumulation, 3) to determine the optimal rate of GA application, and 4) to determine whether two different commercial formulations of GA are equal in growth stimulating activity.
Methods
Experiments related to Objectives 1-3 were conducted in a commercial field of the 'Crimson' variety. Experiment 4 was conducted in the North Willamette Station variety planting on the 'Victoria' W 'German Wine' varieties. For Experiment 1, GA was applied as Gibrel (potassium gibberellate) Gibb-Tabs at 28, 56, or 84 milligrams/crown, 6 crowns/treatment, on January 25, February 8, and March 1, 1983. The GA was dissolved in warm water and injected into a Chapin drip system with 1, 2, or 3 emitters/crown. Approximately 400 milliliters of solution was applied per emitter and was allowed to soak into emerged buds and soil immediately over each crown.
In Experiment 2, GA as Gibrel was applied at 0 or 20 milligrams/crown on January 25, February 8, and March 1, 1983. The GA was injected directly into the crowns at the base of emerged buds. Each crown was injected at three sites with 3.3 milliliters/site of a 2,000 parts per million GA solution or distilled water. Marketable spears from the first two injection dates were harvested on March 15; spears from the March 1 injection were harvested on March 30. Plots consisted of three crowns and were replicated four to six times.
In Experiment 3, GA as Gibrel was applied at 0, 2.5, 5, 10, or 20 milligrams/crown on February 8, and March 1, 1983. In each case, 10 milliliters of distilled water or GA solutions of 250, 500, 1,000, or 2,000 parts per million were applied to each crown, with three injections of approximately 3.3 milliliters/injection site. Spears were harvested on March 15 and March 30, respectively. Each plot consisted of three crowns and treatments were replicated five (February 8) or four (March 1) times.
In Experiment 4, GA as either Gibb-Tabs or Pro-Gibb at 20 milligrams/crown, or distilled water was injected as above on February 2. Each plot consisted of two crowns and the treatments were replicated four times on 'Victoria' and twice on 'German Wine'. Marketable spears were harvested on March 14 and again on April 5.
Results and Discussion
Experiment 1.
Application of GA through the drip system had no effect on growth of rhubarb spears (data not shown). This confirms results previously obtained with spray applications. Apparently, GA uptake is minimal unless the GA can be injected directly into crown or bud tissue.
Experiment 2.
Application of GA at 20 milligrams/crown dramatically increased yields at both the January 25 and February 8 treatment dates (Table 1). In both cases, the major effect of GA was to increase the number of marketable spears (more than 5/8 inch wide, 10 inches long). Average spear weight of the marketable spears was increased very little, if any, by GA treatment. At the March 1 treatment date, GA tended to increase yields slightly, but the increase was not statistically significant. Yield of untreated crowns had increased to a level nearly equal to that of GA-treated crowns for the February 8 treatment date.
Experiment 3.
For the February 8 treatment date, gross yield of marketable spears increased with increasing rate of GA application with very little indication of a yield plateau at high rates (Table 2). However, the yields at 10 and 20 mg GA/crown did not differ significantly. Mean spear weight did not differ among the 5, 10, and 20 mg rates. For the March 1 treatment date, yields tended to increase with increasing rate of GA application up to 5 milligrams/crown, but the yield increases were not statistically significant.
Table 1. Effect of timing of GA application on rhubarb growth response Application Rate of GA Yield of marketable No. of marketable Mean spear date (mg/crown) spears (lb/crown) spears/crown wt. (oz.) Jan. 25 0 0.7 6.8 1.6 20 1.9 16.9 1.8 **Z ** NS Feb. 8 0 0.7 8.5 1.3 20 3.4 29.2 1.8 ** ** * March 1 0 2.9 17.7 2.6 20 4.6 24.2 3.0 NS NS NS Z*, *, NS: difference between means significant at 1% and 5% levels and nonsignificant, respectively. Table 2. Effect of several rates of GA application at two planting dates on rhubarb growth response Application Rate of GA Yield of marketable No. of marketable Mean spear date (mg/crown) spears (lbs/crown) spears/crown wt. (oz.) February 8 0 0.7 8.5 1.4 2.5 2.1 21.3 1.6 5 2.4 20.5 1.5 10 2.9 24.3 1.9 20 3.4 29.2 1.8 LSD(0.05) 0.9 5.3 0.2 March 1 0 2.9 17.7 2.6 2.5 3.6 20.1 2.9 5 4.9 5.2 3.1 10 4.5 24.4 2.9 20 4.6 24.2 3.0 LSD(0.05) NS NS NS Table 3. Effect of GA formulation on growth of rhubarb Yield of marketable No. of marketable Mean stalk Treatment stalks (lb/crown) stalks/crown wt. (oz.) First cuttingZ Check 1.4 9.2 2.5 Pro-Gibb 2.8 16.5 2.7 Gibb-Tab 2.3 14.0 2.6 LSD(0.05) 0.8 4.0 NSX Second cuttingY Check 0.9 7.1 2.0 Pro-Gibb 2.1 13.2 2.5 Gibb-Tab 2.1 13.1 2.6 LSD(0.05) 0.5 2.6 0.3 ZAveraged over 'German Wine' and 'Victoria'. Y'German Wine' only. XNo significant differences.
Experiment 4.
Both Gibrel Gibb-Tab (Merck) and Pro-Gibb (3.91% liquid concentrate, Abbott) stimulated growth of rhubarb (Table 3) and there was no significant difference between formulations when applied at equal rates of 20 milligrams/crown. The growth response of 'German Wine' to GA was greater than that of Victoria regardless of formulation (data not shown). Spears left at first harvest were pulled at a second harvest three weeks later. This second harvest of 'German Wine' was increased by the original GA applications (Table 3), but little response was seen on 'Victoria' (data not shown).
Although GA increased rhubarb yield when applied at each of three calendar dates, the percentage increase in yield was by far the greatest at the second injection date. This was somewhat surprising since the first two sets of treatments were harvested on the same date and the plants injected on the first date had two more weeks to produce marketable shoots. The limited response from the first injection may have been from to the limited bud emergence and poor GA-bud contact in late January. Alternatively, the rest requirement of the crowns may not have been satisfied. For the crowns injected on March 1, it is evident that the rest requirement was satisfied and soil and air temperatures were conducive to rapid spear growth. Control spears grew nearly as rapidly as did the GA-treated spears.
Since climatic differences influence the time required to satisfy the crown rest requirements, GA application on the basis of a pre-determined calendar date is not very useful. Alternative approaches are to inject when the buds have emerged to a certain height or to inject after accumulation of a certain number of chilling units. Correlations of plant response to GA with chilling unit accumulation will need to be confirmed by several years of field observation. In the interim, the best guide to proper injection time remains bud emergence.
There is some evidence in the rate studies (Table 2) to indicate that early application should be at 10 to 20 milligrams/crown and late applications at 5 milligrams/crown. This is to be expected since the native GA in the crowns peaks when the rest requirement is satisfied. Assuming effective injection into buds or crowns, 10 milligrams/crown is probably adequate at any injection date.