Where are the Female Flowers?

Isn't it amazing to see your vegetable garden flush with bright colored flowers of pumpkin and squash. It certainly feels as if one is in the summer wonderland. But wonderland could soon turn out to be CSI (Crime Scene Investigation). Yes, you read it correct, CSI, as one frantically searches for female flowers in this large bright yellow/orange landscape. Now the mystery to solve is: Where are the female flowers?
 
To get to the bottom of this question let us first delve in to the topic of flowering in squashes and other cucurbitaceous vegetables. Cucurbits are 'monoecious', which is derived from a Greek word 'mono', meaning 'single' and 'oecious', meaning 'house'. So in generic terms it means both male and female flowers on the single plant. In cucurbits the word monoecious can be used in other ways too and there are other forms of flowering for example: 

  • Monoecious: separate male and female flowers
  • Andromonoecious: separate male and perfect flowers
  • Gynoecious: female flowers only
  • Hermaphroditic: perfect flowers

Perfect flowers are those that contain both male and female reproductive parts within the same flower. Table 1 provides information on most common flowering attributes of cucurbit vegetables. In monoecious and andromonoecious plants, such as cucumber, muskmelon, summer and winter squash, several male flowers usually open before any female or perfect flowers open. Early in the season as flowering begins, there are usually several times more male flowers than female or perfect flowers. Flowers develop in leaf axils and the stem develops a series of nodes or leaf axils with male flowers, followed by one node with a female or perfect flower, and so on. As the plant grows and matures the proportion of nodes/leaf axils with female flowers increases.
 
Table 1. Cucurbit crops with common flowering patterns

Crop Most common flowering pattern
Cucumber Monoecious
Muskmelon Andromonoecious (monoecious also present but less in proportion)
Pumpkin Monoecious
Summer and Winter squash Monoecious
Watermelon Monoecious (andromonoecious also present but less in proportion)

 
Flowering is not as easy as it sounds as the question of whether a particular node/leaf axil produces a male or female is determined by genetics and environment. In the case of cucumbers, squash, and pumpkins, cool temperatures promote development of female or perfect flowers at a node closer to the base of plant, and the ratio of male to female flowers is reduced. Generally high temperatures promote male flowers, and delay female flower development. Typically for pumpkins, daytime temperatures of 90 degrees F or above and nighttime of 70 degrees F or above lead to abortion of female flower buds. So no wonder, after a spell of hot dry weather, we tend to see more of male flowers in the field or garden. Drought stress can also cause problems with fruit set and cause female flower abortions. Also, dry weather during early growth will cause plants to develop a high male to female flower ratio.
 
It is not only temperature but soil fertility management can also impact flowering in cucurbits. High nitrogen fertilization can also delay production of female flowers. Heavy nitrogen application will often lead to vines remaining vegetative for longer periods of time, producing female flowers only later in the season. This leads to non-marketable fruits as those fruits do not mature in time. High nitrogen availability and lush vegetative growth is likely to occur on heavier soil with high organic matter that are further supplemented with excess amounts of nitrogen fertilizers.
 
So now after a futile search for missing female flowers, when you get back to your shop, house, or shed, you know it is either the weather or perhaps the lure of applying more nitrogen, which has led to those missing female flowers. Well, we hope that the search ends here, as you move ahead hoping for better weather in coming weeks and certainly, not more…not less….but optimum nitrogen next year! 

Authors:

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Yard and Garden, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on July 29, 2016. The information contained within may not be the most current and accurate depending on when it is accessed.