I have three small hosta plants, and one very large one that really needs to be moved. When is the best time to transplant them and how should I do it?
Hosta are a hardy, herbaceous perennial that flowers in the summer, and generally like to be planted either in the autumn, or early spring. The numerous varieties of hosta embrace a wide range of height and width, but all equally enjoy sunlight, and even moderate shade, and will grow on any fertile, moisture-happy garden soil. In fact hostas are so resilient and easy-to grow, that they are often treated like the poor-kid-on-the-block. But, with some vigilant care, their foliages, complex in pattern and hue, and their perfumed flowers, can prove them to be princes in the garden.
Regarding your question about transplanting your young hosta: smaller plants will take about five years to reach full size, and then need no further disturbance for many years. If you really need to move them, then consider that moisture is essential, and mulching with well-rotted organic matter is especially helpful to keep their bed comfortable and nutrient. That is, no stress of dry-gulch conditions.
And regarding your “very large” hosta, the best time to transplant/divide is in the fall, which will give the plant time to re-structure itself into its natural, fully-circular format. If you must deal with this now, in the summer, then lift the clump with a fork, or tip out of the container, taking care not to damage the growing points. Tough, fibrous roots can be gently divided/coaxed into a number of sections, each containing about five or six shoots, then replanted at their original depth in the ground, not too deep. Water the plants, and fertilize with a general-purpose fertilizer, according to instructions. All the best, and enjoy watching your regal plants renew themselves, and thrive in your garden.
Also attaching an informative link to a TMG article related to your query.