Updated: Jun 15, 2021
There is a very rigid tenet when it comes to Permaculture.
It goes: Thine soil shalt not be left bare.
When a patch of soil is left exposed, the sun beats down, penetrating the earth and drying up all water in soil. As a result, the microbes go elsewhere or risk dying, causing plants to fail. Struggling plants will ultimately perish if left to fend for themselves against the hot summer sun.
When we make sure every square inch of soil is covered, more water is retained, more microscopic organisms live, and more plants thrive. This is usually achieved one of two ways. Mulching, or ground cover planting.
There is some nuance to creating a perfectly lush landscape. If you try to grow things as close together as possible, you will see more abundant success. (see Companion Planting)
Creating biodiversity with many layering plants is an incredible way of shading the ground. (see Food Forests)
It is a misconception that plants growing too closely will compete and struggle. While this is true in some cases (sprouts beginning life, invasive species, etc.), most of the time healthy competition for sun will cause many plants to grow bigger.
Many plants actually supply necessary nutrients to the other species around them, thus achieving the harmonic balance so sought after by all Permaculturists alike.
If you can get it to grow, grow it!