Day-Length Effects On Potted Crops By Meriam Karlsson and Jeff Werne

Potted helianthus and rudbeckia vary in response to day-length requirements; find out just how much.

Helianthus annuus and Rud-beckia hirta are popular garden plants with many prospects for use in the landscape. Recent selections with compact growth habits are not only excellent choices for conventional gardens but for use in borders, interiorscapes, containers and even as flowering potted plants. Propagated from seed with similar germination requirements at 70-72° F, these recent crop advancements have successfully inspired new marketing, demand and popularity.

Growth, flower formation and overall crop development are usually satisfactory at any day length for these crops. Growing conditions will, however, affect the expected number of days from seeding to marketing with day length being one of the more significant factors. Shorter days support faster flowering of helianthus, while extending the day will cause earlier flowering of rudbeckia. Although the flower response is opposite in respect to day length, both helianthus and rudbeckia are expected to show less stem growth under shorter days.

The response to day length offers opportunities to manage plant height while monitoring overall crop development. When maintaining a specific climate is difficult, photoperiod may offer an alternative measure for height control. Results on flower rate and plant appearance under day-length-adjusted conditions are presented here for the cultivar selections helianthus ‘Pacino Gold’ and rudbeckia ‘Toto Gold’.

Experimental Procedures

After seeding and germination at 64-72° F, the plants were transplanted into 4-inch pots filled with Premier Pro-Mix BX (Premier Horticulture). The helianthus were transplanted two weeks after seeding and rudbeckia three weeks. Plants were spaced at four pots per square foot and watered once a day with fertilizer solutions of 150 ppm nitrogen using Peters’ 15-16-17 (The Scotts Company LLC). For the helianthus, treatments were initiated at transplant when plants were about 2 inches tall and had five expanded leaves. The rudbeckia plants were only 1?2-inch tall but had seven leaves at the start of the experiment, five weeks from seeding.

The helianthus and rudbeckia plants were grown at constant 56-64° F or 64-72° F. The light intensity was adjusted during the eight hours of short day and the 16 hours of long day in order to provide a similar total daily amount of 8 mol d-1·m-2. The corresponding instantaneous light levels were about 1,050 foot-candles during the 16-hour day and 2,100 foot-candles for the 8-hour day.

In addition to growing plants throughout at short or long days, plants were moved within each temperature to the second day length at weekly intervals for four weeks and left until termination, as illustrated in Figure 3, below. Short day was discontinued after seven weeks, and all plants grew at long days for the remainder of the experiment. Flower rates are reported from start of treatments (14 days from seeding for helianthus and 35 days for rudbeckia).


The helianthus plants flowered more than two weeks earlier at short than long days (see Figure 1, left). The slower development at long days was accompanied by additional leaves, taller plants and larger flowers. On average, helianthus at long days produced eight more leaves prior to flower initiation than at short days. Although temperature had a limited modifying effect on rate of flowering, 68° F at long days resulted in 61?2-inch-large flowers and the tallest plants of 20 inches.

As expected, rudbeckia plants developed very slowly under short days, with up to five weeks quicker flowering at long days (see Figure 2, left). The higher 68° F temperature also enhanced flowering by about two weeks compared to 60° F. While conditions with longer days resulted in almost a doubling of the plant height, flower diameter of rudbeckia remained close to 3 inches independent of temperature and day length. Figure 4, below, illustrates the heights of helianthus grown at short days, long days or combinations of alternate day lengths. Moving plants from shorter to longer days resulted in progressively taller plants, while additional weeks of short days reduced plant height. Similar trends in final plant height were observed at 60° F. The response of rudbeckia paralleled that of helianthus with taller plants under long days.

Transferring rudbeckia to extended days progressively enhanced flowering, and the opposite was true for more weeks at short days (see Figure 5, below). Helianthus exhibited similar flowering trends but with opposite directions, such that long days made for slower flowering. In addition, the flower size was correlated to plant height in helianthus. Reduced flower size in relation to height may be a natural adaptation to certify overall balance and harmony of the plant. Opportunities to produce short helianthus with large flowers are therefore limited using day length as the control method.

Combinations of long days followed by short days appear more suitable for controlling height and growth of rudbeckia than changing short days into long days. To produce compact plants of rudbeckia, at least four initial weeks of short days were required. The disadvantage of an extended initial short-day period is considerably slower flowering. In contrast, rudbeckia was highly responsive to long days for fast flowering. Even a limited number of long days resulted in significantly quicker flowering than continuous short days. Flower appearance was similar for plants grown with four initial weeks of long days to those grown at extended day lengths the entire time. The plants were compact at 7 inches despite exposure to four weeks of long days.

Depending on the intended market for rudbeckia, additional long days beyond the four weeks used here initially may be more appropriate to produce morphologically acceptable plants with fast flowering.

Using The Results

For fast growth and development of helianthus ‘Pacino Gold’, short days are the most productive, resulting in compact plants well suited for containers. Limited early long-day exposure followed by short days can also be used to produce shorter plants. Despite the short-day flowering response of helianthus, limited initial exposure to long days produced rapid flowering similar to uninterrupted short days. Four initial weeks of short days at 60 or 68° F followed by long days are expected to produce helianthus of intermediate height with acceptably large flowers while only needing a few more days than continuous short days for flowering.

Contrary to helianthus, rudbeckia flowered faster under long days, and short days noticeably reduced flowering rate. Although shorter days produce compact plants, several weeks were required to significantly reduce overall height using short days. Arrangements with long days followed by a short-day period appear more appropriate for quick flowering and suitable reduction of height for rudbeckia than do initial short days changing into long days.

Careful use of day length based on the results presented here offers opportunities to understand and use day length for monitoring growth and development of helianthus and rudbeckia. Under natural conditions of increasing day length, slower flowering with reduced height can be expected for rudbeckia compared to fall conditions of decreasing day lengths. On the other hand, less time to flower may be predicted for helianthus under spring conditions of initially shorter days compared to a seasonal change from long toward shorter days.

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Meriam Karlsson and Jeff Werne

Meriam Karlsson is a professor and Jeff Werner is a research professional at the University of Alaska Fairbanks. They may be reached by phone at (907) 474-7005 or E-mail at ffmg[email protected]

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