Tropical what? Plants? In the summer I just stick all my spare aquarium plants in my pond once the water is over 55 degrees. If they're rooted plants, stick em in a pot of dirt and top with gravel, otherwise just fine a way to fit them in.

For fish, wait until it's at least 60-65 at night, at least 80 during the day.

 

If you can I'd bury the containers.... will keep the temperature much steadier then if you had them above ground.

 

Fish. Sadly, burying would not be an option...I will most likely live in an apartment, so even the container pond idea is not absolutely certain.

 

Some take their tropicals out into the pond to reproduce and get better color.
Obvious only when the temperature is warm and stable. 40F at night is bad.

If you can't bury the pond, leave it in the shade and maybe insulate it.

 

Paradise fish are a labyrinth fish that are known for being remarkably hardy. Their labyrinth organ allows them to handle high water temperatures (the warmer water gets, the less O2 it holds), and are also capable of handling cooler temps with ease. There's actually a particular species of paradise fish that requires a period of 55 degree water (or possibly cooler) to thrive. Since they're in the gourami and betta family, you know they're colorful, and while they can also be aggressive, just one in your pond should be fine.

Another (possibly more advisable option) would be to go with native fish. Minnows are always nice, and maybe a dollar sunfish would work?

 

Hehe...I already own paradise fish, so I know of their hardiness and demeanor (they're actually much less obnoxious in harems or groups...they're much more interested in heckling each other than in bothering tank mates). I am hoping to see if the apartment landlord allowed me to keep a small (4-5 ft diameter) kiddie pool from mid May to mid September out on the yard, and try to breed the fish in it. Knowing PF's, they could probably spend half a month out longer on either end, and Macropodus Ocellatus would probably do well as long as I took them in before the freezing began.

Native fish are also an option, yes...but I REALLY like anabantoids, especially paradise fish. Lots of personality, and their interactions are fun to watch.