That planning should begin at the same time as that for the greenhouse itself. The structure along with its contents should be compatible for optimal results. As luck would have it, you have hundreds of choices.
Consider first where you live. Even inside a greenhouse, the quantity of sunshine per day all through the year, humidity and temperature levels along with other variables need to be accounted for.
For instance, Northern Idaho has long, dry summers whereas New Hampshire is much more humid, although it is also in the northern latitudes and gets about the same amount of rain and sun. You can only control humidity to a certain level. If you get loads of sunlight in the summertime and early fall, or can compensate with artificial light sources, greenhouse tomatoes will thrive. The warm temperature makes them grow great and, given that they are easily damaged by frost, they appreciate the protection in the autumn.
Several citrus fruits, such as oranges and lemons, are virtually impossible to grow out of doors in certain locations. It is simply too cold for a good deal of the year. However, you can compensate in the greenhouse and actually grow your own.
Another favorite greenhouse plant to grow are strawberries. Commercial strawberries are pricey and many folks have concerns about pesticides. You will be able to grow your own, control costs and quell health concerns all at the same time.
Radishes, onions, carrots, lettuce and spinach all prefer cooler temperatures, so they can all do really well in a controlled environment such as a greenhouse.
Then, of course, there are the traditional hothouse plants such as orchids. In their natural habitat, orchids grow up higher because they enjoy the breezes. You will be able to imitate that in the greenhouse with a simple fan and ventilating system. Orchids can also be very costly and growing your own can be a great means of saving money while still enjoying beautiful flowers. For the truly ambitious greenhouse gardener, there are dozens of national competitions for orchid growers. However, be careful of getting bitten by the orchid growers bug, which can turn into an all-consuming hobby!
Nevertheless, countless other plants make great choices for greenhouse growing. Just about anything you would grow in an outdoor garden will do fine in a greenhouse, provided you have proper sunlight and shade control. Bear in mind, too, that some plants do not do as well in pots except when the pot is large enough so that the roots have plenty of room to spread.
Most bonsai are outdoor plants, although some benefit from the protection of a greenhouse in high winds and cold winters. Take care that the type you choose is suitable for a high temperature, high sunlight and a humid environment.
Eventually though, it is time to stop planning and to start choosing the best greenhouse plants to grow - Have Fun!