by Leslie Cox; Saturday, July 22, 2017
FYI: The Cucurbitaceae family encompasses cucumbers, summer squashes, winter squashes, pumpkins, gourds, watermelons, and muskmelons.
As a food grower…or grower of food…it is a little alarming to see a healthy plant with lots of flowers but no tiny fruits developing. Especially since summer is progressing and we want to harvest at least a few crops.
The current problem is: all of the flowers are male.
To back up a little…all members of the Cucurbitaceae family have separate male and female flowers. But to confuse matters a little further, cucumber, muskmelon, and squash species are divided into different flower types. These include:
- monoecious (male and female flowers on the same plant)
- andromonoecious (separate male and perfect flowers on same plant)
- gynoecious (mostly female flowers with 5-10 percent being male)
- hermaphroditic (perfect flowers)
Note: Perfect flowers have stamens and carpels, the male and female sex parts, in one flower.
In this article, I am going to talk about just the monoecious curcubit species…the ones which have both male and female flowers appearing on the same plant. Most cucurbits are, in fact, monoecious.
The first flowers to start appearing on monoecious cucurbit species will all be male flowers. Usually there will be 8-10 male flowers on the plant before even one female flower develops.
But depending on the squash or cucumber…or muskmelon…species, the actual ratio of male to female flowers can be as high as 25-30 male flowers to one female, or as few as eight to one…perhaps even lower.
In the interests of harvesting a decent-sized crop, these can be very long odds for the grower. And that is in a good year.
Factor in a few other details…temperature, day length, light levels, nutrition, hormones…and growing zucchinis, cucumbers, winter squash, and muskmelons is enough to try anyone’s patience.
Temperature is your first clue of why you are seeing only male flowers on your cucumber or squash plants. Simply put, high temperatures promote male flower production. If any female flowers should develop on the plant, the high day and night temperatures tend to induce the plant to abort them.
This can be a definite problem if you are growing at least some of your cucumbers in a greenhouse as we are. There are hardly any female flowers on those plants.
However, even my ‘Crystal Apple’ cucumber growing in a large planter on the driveway does not have a single flower…male or female…on it yet! This is my first year growing this particular cultivar of cucumber, so it begs to wonder if the lack of flowers is a programmed genetic delay for this species or hot weather related.
Getting back to weather impacts on cucurbit flower sex development…alternately, cool temperatures will induce monoecious cucurbits to produce only female flowers. So, in cooler climates, or a much cooler than normal season, the tiny fruits shrivel up and drop off because no male flowers have been produced to pollinate the females.
So, in answer to why we are not seeing very many fruits on our cucumber and squash plants…blame the hot weather. Temperatures were too high at a critical period for female flower development…but perfect for male flowers.
I did mention there are other factors which impact on flower sex development in cucurbit species: light levels, day length, and hormones. While none of these have been a primary factor, in most cases, for this year’s crops, I delve into them a little for future reference.
High light levels promotes development of female flowers. Alternatively, too much shade can reduce female numbers, as well as impact adversely on male flower numbers.
Day length, or photoperiod, usually does not impact too strongly on flower sex development…especially for field production. In the greenhouse, shorter days can stimulate more female flowers on some cucumber species.
Plant nutrition also plays a part in the flower sex determination in the Cucurbitaceae family. If nitrogen (N) levels are high…whether in the soil, or through supplementing your plants with a fertilizer high in nitrogen…you will get more male flowers on your plants. Too much nitrogen inhibits female flower development.
Studies have shown potassium (K) plays a role in flower sex development as well. An adequate level of potassium assures female flowers will appear on your plants.
But before you start increasing fertilizer applications, get a soil test done through a lab to see where the nutrient levels, and soil pH, are sitting. Too much nutrient can be as harmful as can too little…and a lot hinges on the soil pH. If pH is too acidic (lower than 6.5), or too alkaline (higher than 7.5), some nutrients become unavailable to the vegetable roots.
As for hormonal influences on your cucurbit plants…think teenagers. They have an impact.
Some hormones are produced within the plant. There are also synthetic growth regulators which can be applied.
Gibberellins are phytohormones (plant hormones) which regulate plant growth and have an influence on several developmental processes such as germination, stem length, flowering, flower sex expression, and dormancy. These hormones promote the development of male flowers in cucumbers and squashes.
Auxins are another group of plant hormones. They are very important in their role of plant growth and behavioural processes…including the promotion of female flower development.
Ethylene gas also has an effect on flower sex expression. It has been found to suppress male flower development and promote the development of female flowers.
This is all heady stuff, I know. But like, I mentioned earlier…much of our current problems of no fruits developing on our cucumber and squash plants is tied up in the weather and there is just not too much we can do about that.
Watch for the next blog on how to tell male and female squash flowers apart. I will also delve into pollination techniques to help the bees and ensure the fruits are being properly pollinated.