2008 Winter – Cornell-copia

Cultural Practices Studies with Cut Flowers, 2007

Sunflower Daylength Screening Test

Executive Summary:  Sixteen sunflower varieties were screened for their reaction to daylength during the seedling stage.  Six showed no reaction in time of flowering or plant size, four showed to be slightly short-day in reaction, and 5 were classed very sensitive, with more than 14 days delay in flowering when exposed to long-day conditions.  Short-day sensitive varieties displayed the formation of axillary buds in the upper leaf nodes when exposed to short days in the seedling stage, a feature which detracts from their appearance.

Many sunflower varieties used as cut flowers are sensitive to daylength.  In our 2006 screening trial, 11 varieties out of 25 flowered in an average of 53 days when exposed to 12 hours daylength in the first 3 weeks after emergence, compared to 70 days if grown under 16 hour days in the same period.  The current trial was conducted to expand the list of varieties tested, and to confirm the results of the previous year.

Materials and Methods:  Seeds were sown in 72-cell trays, placed in either a 12 or a 16-hour daylength in a greenhouse at seedling emergence, and transplanted to the field 3 weeks later.  Field conditions consisted of 4-row beds with 9 x 9 in. spacing, black plastic mulch and trickle irrigation. The experiment was con-ducted three times, sowing May 15, June 15 and June 29.  There were 24 plants per plot, although some plots had fewer plants due to poor germination.

Results and Discussion:  As in 2006, the sunflower varieties showed varied reaction to the daylength manipulation during the first three weeks of growth. Varieties were categorized as day-neutral, slightly or strongly short-day and slightly long-day in their daylength response, and the plant attributes measured followed the expected patterns (Table 1).  Later flowering, due to daylength reactions, allowed more vegetative growth, taller plants and larger flowers.

These results imply that daylength sensitivity should not be a concern if you are growing sunflowers only during the summer in North America.  However, if you plan to produce an early crop in a high tunnel or in a state with mild winter climates, and expect to have them flowering by early June, knowing the daylength reaction of the variety will be important.  Such a crop would be in the sensitive seedling stage in early April, when daylength would be close to 12 hours.  ‘Sunrich Orange’, for instance, would flower early, with short stems, small flowers, and ugly flower buds near the flowering head, as shown in Figure 1.  To avoid such problems, choose daylength insensitive varieties, or extend the daylength to 16 hours using artificial light for the first three weeks after emergence.

General Information:  The studies described below were conducted at Cornell University, at its East Ithaca research farm, where the soil consists of an Arkport sandy loam soil.  The fields in which these experiments have been conducted have received yearly additions of compost.  The high tunnel is in its fourth production season. It was obtained from Rimol, Inc., and has dimensions 98 ft. long, 32 ft. wide and 15 ft. high at the gables.  It is covered by a single layer of clear polyethylene to which IR blocking compound has been added.  The flowers were grown in beds 40 in. wide, and about 4 in. high, spaced about 6 ft. apart and covered with black polyethylene plastic. These were irrigated by two trickle irrigation lines. Unless otherwise stated, plants in the experiments were grown in 4 rows 9 in. apart, and plants spaced 9 in. apart in the row.

Table 1. Reaction of 16 varieties of sunflower to daylength treatments applied during the first 3 weeks before transplanting on days to flower, plant height and flower diameter.

The results of these trials compare well to those conducted in 2006 (see article in Jan. 2007 Quarterly), and confirm the photoperiod reaction of varieties tested in both experiments (Table 3).

Table 2. The effect of seedling photoperiod on flowering date, plant height and flower disk diameter for 16 sunflower varieties, arranged in alphabetical order.

Although at first thought it would seem advantageous to have plants flower earlier after planting, nearly all the daylength-sensitive varieties produced ugly small flower buds in the axils of the upper leaves when given short days during the seedling period (Fig. 1).  This was true of both the slightly sensitive and strongly sensitive short-day varieties, except for ‘Procut White Lite’. ‘Procut Bicolor’, which is categorized as slightly long-day sensitive, did not show axillary bud formation under long-day conditions, indicating that this characteristic is not absolutely linked to daylength reaction.


Fig. 1. ‘Sunrich Orange’ flower after short day  treatment for the first 3 weeks during seedling growth.  Note prominent axillary bud in an upper leaf node.

Table 3. Classification of 37 sunflower varieties according to their seedling response to daylength, as determined in experiments in 2006 and 2007. Varieties are arranged in alphabetical order in each column.