Hand Pollination of Watermelon Flowers
Why on earth would you have to resort to this? Most people with proper gardens don't have to or if they do, they do it because they save seeds and want to prevent cross-pollination. But on a balcony, particularly one high in the air, you're creating a somewhat semi-enclosed eco system that is missing something crucial for the reproduction of many plant species: insects.
Some plants produce flowers that incorporate both male and female parts and are easily able to reproduce on their own. Tomatoes and Peppers are two such plants. They produce what are known as "perfect" flower and all they need to reproduce is a good shake that they can get from a stiff breeze. But in the case of plants that have separate male and female flowers, such as melons or squash, the pollen has to be transferred from one flower to the other via a pollinator. In some cases, the wind can accomplish this but most plant species rely on insects and the one insect most successful at pollinating flowers is the bee. Bees collect both nectar and pollen - sometimes more of one than the other depending on what's going on in their hive. When they visit a flower, pollen tends to "stick" to their little hairy bodies and is carried with them from flower to flower.
Male flowers tend to be more plentiful than the female flowers.
It is easy to tell the difference between the male and female melon flower.
I don't get very many bees on the balcony. Certainly not enough to guarantee that the half dozen female flowers on my lone watermelon plant that only open for one day will get pollinated and turn into melons. So I take matters into my own hands.
First thing to do is know the difference between male and female flowers. Males have a stamen which consists of an anther connected to a filament that emanates from the base of the flower. Female flowers have a pistil which consists of an ovary at the base of the flower that extends along a tube like structure know as a style up to the stigma at the end of which is the receptor for pollen from the male anther. It is the ovary at the base of the female flower that will develop into whatever it is the plant is suppose to produce. And it is the anther on the end of the stamen that carries the pollen needed by the ovary to turn into a fruit.
Female watermelon flowers don't open for very long so it helps to identify them as soon as they appear and be prepared to service them as soon as they are ready. This is typically a one morning affair. When I inspect the plants in the morning and spot a female flower open, I pull off an open MALE flower. Make sure you don't pull off a female by mistake! Don't bother with flowers that look like they are about to fall off or aren't quite open yet. You need flowers that look like they are at their peak: males loaded with pollen, females fully open. With the male flower off the plant, pull off it's petals. I do this to expose the anther which should be loaded with pollen.
You can pull off the male flower petals to expose the stamen, coated with pollen.
Hand pollination is a simple matter of uniformly transferring the pollen from the male anther to the female stigma.
A week after pollination - the beginning of a new watermelon.
With the male flower prepped I carefully wipe what is left of it across the center of the female flower. I try to get pollen all over and around the stigma. Take your time, do a thorough job and do your best not to damage the female flower. If there are more than one suitable male flowers open, grab them and repeat the process on the same female flower. The more the better. That female flower is going to close up tomorrow so you might as well make use of those ready males while they are around.
If all went well, in the case of watermelons, you should notice a big change in the female flower within a few days. The stem attaching the ovary to the vine will thicken and elongate and the ovary will start pointing downward. It will start to get larger at a surprising rate as it turns into a melon.
On my vines I try to get no more than two watermelons per plant. But if you have multiple female flowers ready for pollination, don't hesitate. This is not a sure fire way to guarantee pollination and don't be alarmed if some of your attempts fail. If you end up with too many developing melons, pinch off the weaker ones and concentrate on two or maybe three provided you can supply the plant with the nourishment it needs to support that much fruit.
If you're interested in saving seeds, hand pollination may be the only way you can ensure the seeds from one plant truly represent that plant variety. But you need to do more than just hand-pollination to ensure this. You need to put up some sort of barrier around the pollinated female to prevent stray insects or a good breeze from contaminating the female with pollen from a different flower species. Amy Goldman's books "Melons for the Passionate Grower" and "The Compleat Squash" give fairly thorough accounts on how she hand-pollinates plants in her garden.