Gardening…or is it urban farming? The main reason I “garden” is to grow food to eat. So it’s not gardening in the sense that A) I don’t have my own yard (I live in an apartment) and B) I don’t landscape a space to look nice.
The first rule I can think to share about productive food gardening is to grow what you want to eat. While I have got excited about growing radishes (they are the fastest to grow from seed to harvest), the truth is that I eat very few of them. So planting 30 of them at once is silly.
The second rule is that you may want to grow some things to help the overall health of your garden – ie flowering plants to bring the pollinators to your garden. Not to mention a colourful garden is nice to spend time in.