Answer: Squash and zucchini plants can sprawl over a large area, so bush types of squash are especially adapted to a container.Or vining veggies can be grown upwards on a trellis or nearby fence for support. Plants don't thrive if crowded, so I'd try just one squash/zuke per container and 2 pepper plants per container.
It is important to choose containers that have drainage holes and fill them with a good potting soil rather than garden soil. If you're starting with fresh potting soil you probably don't need to add compost, but if you reuse the soil, do mix compost in. Don't layer the ingredients. Some studies are showing that a layer of stones for drainage doesn't have much influence.
Because the roots are limited by the container, you will need to water more often and apply a liquid fertilizer every two weeks. Or, you can dilute the fertilizer and apply it each time you water to provide a steady flow of nutrients to your plants.
You're right about how hot decks can get. Veggies still need about 8 hours of sun each day to thrive, so if you have an area on ground that will provide them with enough sun, I think it would be a good idea to get them away from that hot deck! Another option is to drape 50% shade cloth over the plants and the deck to keep it cooler.
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