Every week I set out several 3 to 5 gallon planters with strained dirt and start several seeds. Today I started zucchini in a 4 gallon planter. The top is about 15 inches round. I fill the bottom with Styrofoam pieces and cover it with any thing I have on hand like leaves compost or shredded paper. the Styrofoam makes the containers lighter and easy to move.
I put about a dozen seeds and after they get the first set on new leaves I trans plant them. With a big planted I sometimes leave one plant to stay in the planter. With cucumbers I will leave sever to grow with something to climb on. Runner beans are great in planters. I start the seeds all summer and when I get a new spot in the garden I have a replacement that is 10 days or more old! you would be surprised what you can grow in planters and have better control over them! I also like to place a half dozen worms in each planter on top of the leaves or paper. Old straw is great for the bottom over the Styrofoam! I also mix the top 2 inches of strained soil with mushroom compost or sand.