I have six or seven eggplant plants that are growing marvelously. They have been in the ground for about 3-4 months and are each about a foot, foot and a half tall. They all have plenty of purple flowers but I have had no fruit yet. Why? Is it:
Seasonal - I'm in Brisbane, Australia and we are only just now coming to the end of Winter, start of Spring.
User error - do I need to be hand-pollinating these babies?
Nutritional - a lack of food?
Pollination is essential. Check to see what the area has pollinating insects like bees, butterflies or flies. The insects may not be out in full force considering that it is winter, but you would know better than I. If you are fairly certain that you have active pollinators, water may be an issue. If the plant does not have weekly deep watering, the flowers will drop. Make sure that the soil is wet at least 6 inches deep. Mulch to preserve moisture. Water deeply on a weekly basis, more if the climate is dry.
If you have both pollinators and adequate moisture, you could always try pollinating by hand using a fine paintbrush or some cotton swabs. Just lightly transfer pollen from the flower of one plant to the flower of another. Let us know what happens. Often wnd can be a good enough pollinator for these plants. (If there is enough wind)
I think as spring starts to arrive and things warm up, this plant will take care of it self.. (patience)
Advice for aiding pollination is to gently tap the flowers two or three times a week until you see they're pollinated, repeating this if necessary. Either that or use a Q tip or old eye make up applicator to lift pollen from the stamens (the filaments in the middle of the flower) and apply it to the tip (stigma) of the pistil in the centre. The flowers are 'perfect', meaning there aren't separate male and female flowers - each flower, if pollinated, has the capacity to turn into a fruit, and each individual flower can pollinate itself, it does not need to be another flower's pollen.
Eggplants can be sensitive to temperature. If it's too hot, they may not set fruit, probably because of sterile pollen (from the heat) as with tomatoes. My Diamond eggplants and another unknown variety wouldn't set in the heat (despite numerous flowers that were not in the habit of dropping particularly), but they would set fruit when it cooled down. However, if it cooled too much they wouldn't set fruit then, either. If you live in or near the desert you'll likely have a particularly hard time because of the major shifts in temperature between day and night.
I propose planting cold-tolerant and heat-tolerant eggplant varieties. Don't plant them in the shade. Apple Green is cold-tolerant. Aswad is heat-tolerant. Malaysian Dark Red might be both. Locally adapted varieties are important, in my opinion. Saving seeds every year rather than buying new ones from somewhere else may help them to adapt more.
Looking at what the Internet says about Brisbane, I'm guessing if you have temperature issues they would be with it being too cool at night. I don't know how cold it gets at night, though.
See this Q/A, which is related to your question and my answer.
It's also quite possible that there's an issue with your soil. I know you're not talking about tomatoes, but they are closely related: I grew the same varieties of tomatoes as my neighbor (from the same seed packets, even). The only notable difference in growing conditions was the soil. My neighbor's tomatoes grew huge plants fast with a whole bunch of flowers (but the flowers wouldn't set fruit for a long time, it seemed). My plants were much smaller and had fewer flowers, but had more fruit earlier on. I'm guessing my neighbor's soil had more nitrogen and mine had more potassium (I gave mine a fair amount of potassium sulfate, rock phosphate and basalt rockdust). I don't think my neighbor amended her soil. Last year she had plenty of hay about; so, that may have left some nitrogen in the soil. If it weren't for the heat, I'm guessing her numerous flowers would have set fruit much earlier. Potassium does provide some heat-tolerance, I've read. Phosphorus helps with cold-tolerance.
