By Russell T. Nagata
By Russell T. Nagata
University of Hawaii
at Manoa College of
Tropical Agriculture and Human Resources
Komohana Research and
Extension Center
“Why don’t I have fruits on my garden vegetables and fruit trees?” is a commonly asked question.
While there is no one correct answer — as it could be related to a number of causes ranging from plant age and physiology, nutritional status, disease and pests, growing environment or lack of compatible partners — poor pollination is always suspected.
Unlike leaf and root crops, where the lack of pollinators in our gardens may go unnoticed for quite some times, lack of fruit set will be noticed fairly quickly. Fruit development in nearly all cases is the result of fertilization and subsequent seed development. In certain plants, bees and other pollinators transfer pollen from the anthers to the stigma, resulting in fertilization.
An exception to pollination and fertilization for fruit development is parthenocarpic fruit, where fruit development occurs without fertilization. If you suspect that you are not getting fruit set due to the lack of pollination, it’s time to do a little investigating and experimenting.
Flowers are the first things to check.
The presence of perfect flowers, those having male (pollen producing) and female (seed producing) parts, or male and female flowers either on the same plant (monocious) or different plants (dioecious) are essential. It is also good to use a magnifier to check for the presence of pollen on the anthers. Do a little research and find out about the pollinators for your crop and whether they are present in your location.
The fastest method to determine the presence of pollinators is to look for them. You can also hand pollinate the flowers to complete the task that pollinators do. If fruit set occurs, it is a good indication that lack of pollinators and pollination was responsible for the lack of fruit set.
As with all new jobs or tasks, you will need to learn what you have to do to be successful. For the job of pollinator, you will have to know when pollen is available, where it is found and where to put it. When you do your job well, you are rewarded with a cornucopia of really great fruit. Timing as well as the amount of pollen transferred will have an effect on the out come. The amount of seeds that develop within the fruit may also affect the size of the fruit, more seeds lead to larger fruit. Even in the case of seedless watermelons where the lack of seeds is the whole point, the initial pollination is critical to enjoy watermelons on a hot summer day.
As an example, squashes, pumpkins, watermelon and cucumbers are all naturally monoecious plants where each plant have both male and female flowers that require pollinators to carry pollen from the male flowers to the stigma on the female flower.
Step No. 1 is to identify the male and female flowers. For all of these crops the female flowers have a miniature fruit attached below the petals. These immature fruits tend to be fuzzy as there are covered with trichomes (hair like fuzz). The male flowers only have a thin stem below the flower and shed pollen from the anthers.
Pumpkins, squash, cucumbers normally shed pollen in the morning, when bees carry them from flower to flower. Others, like white squash, shed pollen in the afternoon, awaiting pollinators that arrive at dusk or at night.
Pumpkins and squashes can be hand pollinated using the following procedure. Get to know your male and female flowers and where you can find them on the plant. The night before pollination, select the female flowers that you will be pollinating the next day and secure the petals closed with a clothes pin. Select one to two male flowers for every female flowers from which you will obtain pollen and also pin those closed.
Pinning the flower closed serve two functions. It insures that moths and other non pollinator insects will not disturb the pollen before you are ready to pollinate and protect it from rain. Shortly after sunrise locate your female flowers and remove the clothes pin and open flower. Cut the male flower from the vine leaving a long stem on the flower. Carefully cut the petals off right above the sepal using a sharp knife and gently remove the petals without disturbing the anther and pollen. Holding the stem of the male flower like a brush, transfer pollen from the anthers to the stigmatic surface of the female flower.
Normally the stigmatic surface will resemble a shiny three part structure in the middle of the female flower. Pin the petals closed on the female flower and observe over the next few days. If your pollination was successful, the fruit will soon begin to grow larger.
For more information on this and other gardening topics, please visit the CTAHR electronic publication website at http://www.ctahr.hawaii.edu/Site/Info.aspx or visit any of the local Cooperative Extension Service offices around the Island. I can be reached at russelln@hawaii.edu.