Splash out on a few grafted plants.
Grafted plants have been, well, grafted on to different rootstock, which means they grow more vigorously, fruit more quickly and heavily and have greater disease resistance. Various veges are sold as grafted plants: look for grafted tomatoes, eggplants and watermelons. The plants are more expensive but, because they fruit sooner, great if you want to grow a crop that’s marginal in your climate. Even in my Auckland plot at the warmer end of the country, I find the bigger purple eggplants can be hit and miss when grown on their own roots, whereas grafted plants all but guarantee you a crop. And because you’ll get a heavier yield of fruit, grafted tomatoes are a great choice if you only have room for one or two plants.
Try grafted eggplants.