Ultimately, this is based on your preference. Most plants will grow in close proximity to one another. Planting them close together prohibits the plant from expressing their true botanical form. This may be preferred if you are attempting to form a screen or hedge. If the preference is not to have the plants touch each other when they reach maturity, first determine the mature spread of the plant in question. The mature spread of each plant is displayed on each plant product page and plant tag.