Carrot seed research in the Pacific Northwest

Here is a stand of carrots. In this photo the umbel structure is clearly visible.
Photos courtesy of Rhonda Simmons, OSU.

In central Oregon, where 75 percent of North America’s hybrid carrot seed is produced, almost half of the state’s seed growers irrigate their crops using drip irrigation. The remainder use either sprinkler or furrow/flood irrigation. Recent anecdotal evidence suggests that the method of irrigation has a pronounced effect on germination.

Because yield and germination percentage are the key determining factors in seed production profitability, researchers at Oregon State University conducted a two-year study of the effects of irrigation methods on germination of carrot seed. Rhonda Simmons led the study, the first phase of which she completed in the spring of 2010. Different carrot seed varietal germination rates vary with environmental stimuli, so the research team conducted their trials on a hybrid carrot seed that has grown in the area for several years. Simmons intended to help both growers of carrot seed and commercial carrot growers by creating methods to produce higher-quality seed with better germination.

Seeds with higher germination rates require fewer plantings to achieve high stand. The term “stand” describes emerging seedlings in a field. Seed growers must utilize good quality seed and best planting practices in order to produce a uniform field with high stand, and without dead or missing plants. Areas with missing plants provide favorable space for weeds to grow. This can be detrimental for carrot seed growers if the seeds produced from weeds are a similar size to carrot seed, because it makes cleaning the carrot seed more difficult and more expensive. Farmers have to spend more money and time weeding and spraying to protect their crop.

Simmons says prior to the OSU study, work regarding environmental effects on the germination percentage of carrot seed was limited. Although some studies have determined the effects of temperature and drought stress during seed fill on various crops, there is little information regarding humidity and its effect on germination percentage of any species.

Daucus carota L. grows in an indeterminate manner, producing seed in umbels that arise from multilevel ordered branches. Bees pollinate each of the many seed-producing flowers. The center inflorescence, known as the primary umbel, appears first and is generally the larger, topmost umbel of the carrot stalk. Subordinate branches of secondary, tertiary and quaternary umbels, named in relation to their appearance on the plant below the primary umbel, develop throughout the growing season. The secondary, tertiary and quaternary umbels all radiate from the stalk of the primary umbel. The size, vigor and germination of carrot seeds, which determine the seed quality, vary according to the umbel order.

At the onset of the OSU irrigation study, Simmons noted a difference in the germination percentage of carrot seed when grown under different irrigation treatments. She theorized the possible reasons for these differences:

1. When the umbel of the mother plant gets wet, localized relative humidity is higher and leads to fungal infections and a reduction in germination percentage.

2. Drip irrigation events are more frequent than sprinkler irrigation events, thus offering the plants more moderate wet/dry cycles, and reducing the stress on the plant that could be caused during a flood/drought situation caused by sprinkler irrigation.

3. The irrigation method alters the yield composition, resulting in an increase in the number of umbels that produce immature and sterile seeds.

A bee pollinates a carrot umbel.

At the time of this writing, Simmons has yet to compile the results of the OSU study formally, but she reports consistently higher rates of germination in carrot seeds in drip irrigation. Although she and her colleagues don’t have a clear idea why they’re seeing an increase, Simmons notes:

1. A slight increase in the germination of the primary (or king) umbel.

2. The secondary and tertiary umbels are producing “higher-germinating” seed, which increases the overall germination.

Simmons intends to investigate why the latter-appearing umbels are producing higher-germinating seed in subsequent studies. She wonders, “Could it be the plants are healthier due to better water usage? Do we get better pollination because the bees can continuously work the lower umbels in drip as opposed to having to work around the overhead sprinkler? We don’t know the answers at this time and are working with colleges to determine what direction we should go to find the answer.” 

As she continues to investigate factors that might contribute to in the increase in germination, Simmons will work with local industry groups and fellow OSU researchers located on campus.

Learn about OSU studies on carrot seed health in next month’s column.

The author is a freelance writer based in Massachusetts and a monthly contributor to Growing.