Pinching Sunflowers to Increase Number of Flowers per Plant and to Control Flower Size

When we first thought about doing a short article about pinching sunflowers, I thought we might be wise guys and title the article “To pinch or not to pinch, that is the question” with apologies to all our literary friends. However, that is not really the question. The real question is “What cultivar to pinch and when?” Although we do not have all the answers, we do have some useful information on pinching sunflowers we want to share.
    
There is a lot of interest in pinching sunflowers because the market you may be trying to grow for might require smaller blossoms, especially for bouquet work. Clearly, most standards (single flower on a single stem) are just too big for inclusion in bouquets with smaller flowers. Stevens (1997) stated that for yields of sunflower you could expect one terminal flower and three to five axillary flowers from each mature plant. He further noted that the terminal flower will have the longest stem length, while the axillary or lateral branches may have stem lengths that are too short.
    
Similarly, Dole and Wilkins (1999) stated that the primary (terminal) flower is the largest and is generally of higher quality with longer stems than the axillary (lateral) shoots. For smaller flowers, an alternative would be to grow small-flowered types such as ‘Sonja’or ‘Soraya’ but they do not provide the classic look of a ‘Sunbright.’ Typical of the interest in pinching is a question by Paula Grote on the ASCFG Bulletin Board. She asked “When is the best time to pinch the branching sunflowers? I am planting several and have heard the fifth leaf, but would someone with experience growing them tell us the best time?” Everett answered “Your question is a good one, but more complex than it first appears, because the answer changes with the time of year, spacing, the cultivar you are growing and the product you are trying to produce. In general, however, sunflower, respond to pinching when they have about 3-4 sets of true leaves, giving several (2-5 or more) stems per plant with stem length suitable for bouquet work. Later pinching yields more, but usually shorter, stems. Pinch a few at different stages and growth and see how they do.”
    
Complex Growth Habits

Cut sunflowers cultivars are complex in growth habit depending on the cultivar. Some are short, others very tall, some have many lateral branches and others have few or none. Further, you can “grow” flower size by implementing a number of cultural practices, including selecting small-flowered types (‘Soraya’ among our favorites) or you can increase plant density and get more, smaller flowers. The former may not give you the flower form or color you want, and the latter will still have some that will grow into large flowered plants. You will get a variable crop if you are growing in rows or beds, especially if it is a vigorous cultivar. However, if you restrict soil volume in containers with a high density planting you get more predictable results, but with very demanding cultural requirements. Pinched plants will generally produce more flowers per plant with shorter stems and smaller flowers, but as always, it depends on the cultivar.

Lateral Branches

Another component of the complex growth habit is the tendency for some cultivars to have many lateral branches and others very few. For some cultivars, to grow them as standards you must remove all the lateral buds when they are small, which allows the terminal flower to develop into desirable stems for the wholesale market. ‘Lemon Aura’ is an example of that type. In addition to the terminal flower, there are many short, unmarketable lateral stems which must be removed, the earlier in the growth cycle the better. However, removing the terminal bud of ‘Lemon Aura’ to encourage these laterals to grow would produce only short-stemmed laterals of questionable market value. Another type that may be called a spray type or multiflora would have many longer-stemmed laterals. An example would be ‘Velvet Queen’, a tall cultivar with strong lateral growth and long-stemmed laterals. Getting a good terminal flower on this cultivar would involve removing the lateral buds, but in this example all those lateral stems have potential marketable flowers that would be lost. Dole and Wilkins (1999) noted that multiflora cultivars are especially well suited to being pinched and recommended pinching at the four to six pairs of leaves.
    
Cultivar

Some cultivars tend to branch more than others. When grown as a standard, they can get too tall and do not make a quality marketable stem, while the lateral branches are excellent cut flowers. Others were selected for the standard cut flower market with a nice single terminal flower and the lateral branches, if any, are too short for a marketable flower. Some short cultivars do not lend themselves to cut flower production and are best used as bedding plants or flowering pot plants. We grew 42 different in a cultivar trial and they can be viewed on the web at http://hort.ifas.ufl.edu/floriculture/specialty_cut.htm Click on “2003 Sunflower Trial”
    
Time of Year

Some cultivars vary in the number of lateral buds with the time of year. For example, ‘Sunbright’ will have very few lateral buds in the spring, summer and early fall, but during the late fall and winter months we noticed many lateral buds that must be removed as the terminal develops, to have just one flower per stem.
    
Experiment

With those observations we decided to pinch a few ‘Sunbright’ at different stages of growth. In addition to Paula’s question above, we wanted to get some fundamental information for  field growers in south Florida who are trying to produce the ‘Sunbright’ look but with smaller flowers, for the bouquet market in Miami. Although increasing planting density partially accomplishes this, it does not produce a uniform product for their market.
    
This is what we did. ‘Sunbright’ seeds were planted on June 30, 2003 in Gainesville, Florida in the Specialty Cut Flower Research Program facilities, at 15 cm (6 inches) x 20 cm (8 inches). Grew plants for three weeks and pinched them. They were about 9 cm (3.5 inches) tall with about 3-4 sets of true leaves at that time. The next week (week 4) we pinched a new group of plants and they were about 23 cm (9 inches) tall with more leaves, then again the next week (week 5) even taller at 48 cm (20 inches) and finally the next week (week 6) at about 74 cm (30 inches) tall.  We grew them with the same culture as those that were not pinched and grown as single stem plants.
    
We then counted the days to flower and measured stem length. The results are presented in Figure 1. Interestingly, the days to flower (expressed as AFD, average flowering date) was very similar for all pinched treatments, with a 73-78 day range.  However, the stem length was the greatest  from the early pinch date (about 36 inches), while the last pinch date yielded stems too short (about 12 inches).  The pinch at week five averaged 20-inch stems while the pinch at week four averaged 30-inch stems.  ‘Sunbright’ grown adjacent to the pinching study and planted on the same date, although not part of the experiment, flowered at 72 days and 160 cm (over 5 feet). Thus, pinching resulted in a very slight delay in average date to flower and reduced marketable stem length and observed flower size.
    
Although the data suggest that stem length of 20 and 30 inches (Pinches B & C) would be suitable for the latter pinches, it does fully describe the results. The pinch at three weeks (Group A) produced  plants that were fairly uniform, with few deformed flowers, of a size for bouquet work.  Group B at four weeks might be considered commercial quality but were more variable in both stem length and flower size. Groups C and D just did not have nicely formed flowers or consistent stem length. Variability in stem length and flower size and quality was not as desirable.  
    
‘Sunbright’ was the only cultivar we pinched, but the plants pinched early in their development resulted in a nice crop of uniform flowers on long straight stems suitable for bouquets. Additional work needs to be done on pinching, and we plan on including more cultivars this coming season including those cultivars that are described as branching types or multiflora that Paula asked about.

Sunflower Pinching Experiment

Figure 1. A chart showing stem length and average flowering days (AFD) from pinching ‘Sunbright’ plants at four different times from a direct seeded planting on June 30, 2003.  As pinching was delayed in the growth cycle stem length of lateral branches decreased.

Summary

Pinching sunflowers is a practice used to increase the number of flowers per plant, reduce flower size, and manage lateral stem length, by selecting the stage of growth plants are pinched. With ‘Sunbright’, time of flowering was only slightly increased over similar plants grown as standards. However, the number of stems produced increased per area of bed space, stem length compared to standard culture was decreased, and when pinched early, plants had an acceptable stem length for bouquet work, with the corresponding decrease in individual flower size.

Literature Cited
   
1.  Dole, J. M. and H. F. Wilkins. 1999. Floriculture: Principles and Species. Prentice Hall, Upper Saddle River, New Jersey 07458.
2.  Stevens, A. 1997.  Field Grown Cut Flowers: A Practical Guide and Sourcebook.  Avatar’s World, 106 E. Hurd Road, Edgerton, Wisconsin 53534