What method helps maintain the size of upright junipers and arborvitae plants?

Maintaining the size of upright junipers and arborvitae primarily involves annual pruning. This method is effective because it allows for the controlled removal of excess growth, which can help to keep the plants within a desired size. Pruning also encourages denser foliage growth and improves the overall shape of the plants.

While seasonal watering, fertilization, and mulching play important roles in the overall health of the plants, they do not directly impact the size maintenance. Seasonal watering ensures that the plants receive adequate hydration, which is vital for their growth but does not restrict their size. Fertilization provides necessary nutrients to promote healthy growth, and while it can influence the vigor of the plants, it doesn't help manage their size directly. Mulching can help retain soil moisture and suppress weeds, contributing indirectly to plant health, but again, it does not control size. Therefore, annual pruning is the key practice for managing the size of upright junipers and arborvitae.