How deep do watermelon, cantaloupe, summer squash & cucumber plant roots grow? How big of a pot do I need for above ground gardening on these plants?
5 to 20 gallons (I would recommend 10 to 20) should be good, depending on several factors (don't put more than one plant in a 5-gallon container). 30 gallons is probably overkill. I wouldn't do less than 5. The smaller the container, the more often you'll need to water (and the greater the chances are that your plants will be damaged when they dry out). The smaller the container, the more you'll need to fertilize (but you should still fertilize with larger containers).
Smaller containers result in smaller plants. The bigger the plant is supposed to be, the bigger the container should be (unless you're trying to dwarf it by keeping it in a smaller container, and don't mind watering and fertilizing it a lot). However, plants dwarfed by being in a smaller container will not only have smaller leaves, but will have fewer leaves, and probably produce less; they may have less dry matter in their stems, too. See this PDF for a source on the information in this paragraph.
You should ensure that you use a good quality soil, and that you fertilize properly (and regularly).
Here are a couple links that give recommendations on container sizes:
- https://savvygardening.com/growing-watermelon-in-containers/#:~:text=Choose%20a%20pot%20that%20holds,Remember%2C%20that's%20a%20minimum. (This recommends 7 to 10 gallons for Bush Sugar Baby watermelons, and bigger for other kinds.)
Bush-type plants are often recommended for containers, but if you don't mind long vines, and perhaps a somewhat to much larger container, then you should be fine with non-bush types.
There are smaller and bush-type cantaloupes/watermelons, but there's not a wide variety of them, unfortunately (most melons grow large vines). There are Bush Sugar Baby, Bush Jubilee, Minnesota Midget, and some others.
There are other bush-type squash than zucchini. Trivoli Spaghetti F1 is an example of a winter squash (I don't remember the other bush-type summer squashes offhand).
As far as how deep the roots grow goes, see RubberDuck's answer here. However, note that watermelon roots can technically get deeper than that. I haven't noticed them being terribly deep in my own garden, though. I think there was this guy who had a bunch of old horse manure who got his watermelon to grow 5 feet deep (but I'm not sure if I'm remembering that right).