J. Raymond Kessler, and Gary J. Keever

Achillea × ‘Coronation Gold’ is a widely grown garden perennial with yellow flowers that blooms in early summer at a height of about 2.5 feet. Achillea filipendulina, one of the parents of this cultivar, has an obligate requirement for 8 weeks of vernalization at 41^oF followed by long photoperiods (>14 hours or night-break lighting) for complete flowering. Achillea × ‘Coronation Gold’ responds in much the same way, but its flower stems grow too tall in containers under greenhouse conditions for market acceptance; therefore, it may benefit from plant growth retardants. This investigation was conducted to determine the plant growth retardant type, rate, and application timing required to produce a marketable greenhouse pot plant of Achillea × ‘Coronation Gold’ grown in 6-inch pots.

METHODS
Vegetatively propagated offsets of Achillea × ‘Coronation Gold’ grown in 72-celled flats were transplanted into 6-inch plastic pots containing Fafard 4-P. Plants were pruned to 1 inch above the substrate at the time of planting. Plants were then placed pot-to-pot in a plastic covered greenhouse under naturally occurring short photoperiods with a heating set point of 62^oF and ventilation at 78^oF. Fertilization throughout the experiment was applied weekly at 150 parts per million (ppm) nitrogen using a 20-10-20 fertilizer. Plants were watered/fertilized when the medium appeared dry, but before plants wilted. After 7 weeks growth, all plants were placed in a walk-in cooler at 40^oF for 9 weeks. Incandescent light (60 watt) was provided at a minimum of 10 foot-candles for 24 hours per day and clear water was applied as needed while in the cooler.

After cooler treatment, plants were placed in a glass-covered greenhouse with a heating set point of 65^oF, and ventilation at 76^oF. Plants were provided long photoperiods beginning 1 week after removal from the cooler until the end of the experiment by lighting from 10:00 p.m. to 2:00 a.m. using a minimum of 10 foot-candles from incandescent lamps (60 watt). Plant growth regulator treatments were applied with a pressurized CO₂ sprayer set at 20 psi at a rate of 0.5 gallon per 100 square feet. B-Nine at 0, 2,550, 5,100, or 7,650 ppm; Bonzi at 0, 32, 64, or 96 ppm; Royal Slo-Gro at 0, 450, 900, or 1,350 ppm; or Sumagic at 0, 10, 20, or 30 ppm was applied 2 weeks, 4 weeks, or 2 and 4 weeks after removal from the cooler using half the concentration.

After treatment, plants were spaced on 12-inch centers. Data recorded at the time of first open flower was flower date, shoot height, growth index [(height + width1 + width2)/3 where width1 is at the widest point, and width2 is perpendicular to width1], a market quality rating (1=very poor, unsalable; 2=poor, unsalable; 3=average, salable; 4=good, salable; 5=excellent, salable), and the length of the five longest lateral shoots. Flower date and shoot length were recorded when each flower head on a plant opened. A flower head was defined as open when 90% of the florets had expanded yellow petals. Once all the flowering shoots on a plant were open, the number of flowering and non-flowering shoots was recorded and a market quality rating (1=very poor; 2=poor; 3=average; 4=above average; 5=excellent) was assigned.

RESULTS
There was no effect of application timing. All growth retardants resulted in decreased shoot length with increasing concentration except Royal Slo-Gro (see table). Applications of Royal Slo-Gro resulted in delayed flowering, highly variable shoot lengths, and low market quality ratings. In addition, Royal Slo-Gro reduced the number of flowering shoots (data not shown), produced foliar chlorosis, and distorted flower heads. The highest rate of B-Nine, Bonzi, and Sumagic decreased shoot length by 32%, 19%, and 22% compared to untreated plants, respectively. The highest rate of B-Nine, Bonzi, and Sumagic increased the time to flower by about 4 days compared to untreated plants while Royal Slo-Gro delayed flowering by more than 22 days. Market quality rating for plants treated with Royal Slo-Gro was low, averaging 2.0. Market quality rating increased with increasing concentration for B-Nine, Bonzi, and Sumagic with the highest rating, 3.6, given to Sumagic at 30 ppm.

Response of Achillea × ‘Coronation Gold’ to spray applications of B-Nine, Bonzi, Royal Slo-Gro, and Sumagic
Growth retardant	Rate (ppm)	Days to flower	Flower shoot length (cm)1	Market quality rating2
B-Nine	0	43	46.7	2.0
	2550	44	41.1	2.0
	5100	46	36.8	2.7
	7650	47	31.7	3.3
Bonzi	0	42	46.2	2.2
	32	43	43.7	2.3
	64	43	40.9	2.9
	96	46	37.4	3.0
Royal Slo-Gro	0	43	47.1	2.0
	450	58	44.2	2.0
	900	63	43.6	2.0
	1350	65	46.8	2.0
Sumagic	0	42	46.9	2.1
	10	44	42.7	2.3
	20	45	42.1	2.8
	30	46	36.4	3.6
1 English conversion 2.54 cm = 1 inch. 2 Market quality rating: 1=very poor; 2=poor; 3=average; 4=above average or 5=excellent.

B-Nine, Bonzi, and Sumagic at the highest rates resulted in sufficient shoot length control in Achillea × ‘Coronation Gold’ for it to receive average market quality ratings of 3.0 or higher (average and salable). A flowering delay of about 4 days would probably not be a significant problem in commercial application. However, labeled rates for Bonzi and Sumagic are well below the rates found effective in this study. It is probable that the cost of applying Bonzi or Sumagic at these high rates may be prohibitive. Therefore, B-Nine at 7,650 ppm may be the most cost effective plant growth retardant to use for greenhouse production of Achillea × ‘Coronation Gold’ in 6-inch pots. Quality of plants treated with Royal Slo-Gro was unacceptable.