Answer: You could certainly use it as a planter if it has a way to allow excess water to drain out the bottom. Most plants will not use more than about a foot or foot and a half of soil so you might use a filler (styrofoam peanuts or aluminum cans are possibilities) at the base rather than using all soil, but cover the filler with a layer of landscape fabric or similar material to keep the soil from washing down into the filler layer. This will also help when it comes time to replace or renew the soil.
You might also consider using it as a water garden since you know it is strong; simply add a pond liner and water along with plants -- there are both hardy and tropical water plants available.
In most cases a greenhouse supplies ample natural light in all but the cloudiest of years, but temperature is an important consideration. If the greenhouse is heated you could conceivably grow plants during fall, winter and spring. It may however overheat on cold but sunny days and especially during the summer. There should be an automatic vent system as well as a means of shading it during the summer along with good air circulation. In any case you could use it as an unheated cold frame in fall and spring for growing cool season crops such as leafy greens, peas, radishes, and parsley and for starting off many garden plants in the spring.
Experiment and enjoy that hot tub!
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