Why don't my pumpkins have any pumpkins?

Pumpkin plants are easy to grow and popular with many food gardeners.  However, it is quite common for people to become somewhat worried as the season progresses.  'My pumpkin plants have gone berserk.  They now take up this really large area, they look incredibly healthy, I have seen many flowers, but I don't see many pumpkins.  What's wrong?'

Pumpkins are part of the Cucurbit family (Latin Cucurbitaceae).  This plant family includes pumpkins, squash, cucumber, zucchini, watermelon and cantaloupe.  All the members of the family are bee-pollinated (ie. not self-pollinated, wind-pollinated or insect-pollinated).

Aha, so here is the first possible answer to your question: if there are no bees, there will be no pollination, and no pumpkins.  The absence of bees is indeed a problem in some parts of the world, but luckily not in Tasmania, so it is not likely that this is the answer to your problem.  In most gardens these miracle workers just turn up out of nowhere and do their job.  Amazing!

You say that your pumpkin plants have a lot of flowers, but few pumpkins.  Okay, let's have a look at those flowers.  Flowers on members of the Cucurbit family are either male or female, and they are easy to tell apart if you know what to look for: 

left- male ; right - female

In the photo above the left-hand flower has a straight stalk and there is nothing between the stalk and the flower.  It is a male flower.

The flower on the right-hand side is a female flower.  Between the stalk and the flower is a thickened area which is an embryo.  Once a female flower is pollinated, its petals fall off, and the embryo begins to grow.

The photo below shows the difference between male and female flowers when you look into the flower.  If you peel away the petals of a male flower (as was done in the photo) you find what is called a 'stamen'.  When the male flower is ready for pollination the stamen will be covered with a lot of yellow pollen.

Chances of natural pollination are increased dramatically by not watering Cucurbits overhead, because, if you do, you may wash (some of) the pollen off the stamen of the male flowers. 

flower head with petals removed - male ; flower with petals - female

The female flower has a 'stigma', which must be pollinated for fruit to develop.

So now have a look at all the pumpkin flowers in your garden.  How many are male and how many are female? 

Cucumber flower - female

Cucumber flower - male

 

About the ratio male/female flowers on Cucurbits:

• When members of the Cucurbit family begin to flower, male flowers appear first.  Female flowers appear further down branches and begin to appear up to a fortnight later.  Some gardeners become concerned too early!
• Pumpkins (and many other varieties of Cucurbits) do not like hot, humid weather.  Plants may hold off producing female flowers in the hope that the weather cools down.
• Too much nitrogen in your soil can result in mostly male flowers.
• Some hybrid Cucurbits produce few female flowers.  It may be a reason to avoid hybrid Cucurbits.

This is where we get to the subject of 'hand-pollination'.  In many cases you can trust nature to do its thing and all will work out, but, if you have a Cucurbit variety that produces few female flowers or they are scarce for one of the other above-mentioned reasons, hand-pollination is the way to go to make sure that every scarce female flower results in a fruit. 

Hand-pollination is also advisable if temperatures during flowering are too high (over 30 Degrees C) or too low (under 13 Degrees C) because the bees will probably take the day off. 

Hand-pollination of Cucurbits is easy if you observe a few rules:

1. Watch the female flowers.  Look out for a day when a female flower is about to open.  It will be open only on that day, and only for a few hours in the morning.
2. Hand-pollinate before 10am, because later in the morning temperatures may go up and pollen may be too hot to get good results.
3. Select a male flower ready for pollination.  Touch the stamen with your finger and see if yellow specks come off on your hand. Only if this is the case the pollen is ready.
4. Pick the male flower, remove the petals, then gently rub the pollen onto the stigma of the female flower, see second photo (instead you can use a soft brush or cotton-bud stick).  Be sure to come in contact with all segments of the stigma. 
5. The female flower will close later in the morning and a Cucurbit fruit is on its way.

Well, fingers crossed, because fruit may not set if:

1. The plant does not get 6 - 8 hours of sunlight.  If possible, always plant Cucurnbits in full sun.
2. The plant does not get enough water, so please observe!
3. Temperatures are too high or too low.  Plant stress may cause the fruit to abort as a result.

You may wonder what happens if you have pumpkins, cucumbers and zucchinis in close proximity.  Will they cross-pollinate?  Will you get pumpcumbers and cucumchinis? 😅😅 

The answer is 'no'.  Cucurbits do not cross-pollinate, but varieties within one species (eg. two varieties of pumpkin) certainly can, so keep that in mind when you choose your Cucurbits for the season.

May the bees be with you!