The cheapest and easiest way to increase your number of hosta plants is to divide mature individuals. Hostas can be successfully divided anytime they're actively growing, and they're very forgiving when it comes to almost anything, including division. Some gardeners prefer propagating in early spring before foliage leafs out fully and the soil is still cool and moist. Others wait until mid- to late summer. The best candidates are 3- to 4-year-old plants, which tend to produce the largest and best divisions. Older clumps are often quite large, require a major effort to lift, and typically sustain more damage to individual plant crowns and roots. With a little practice, you'll soon be producing divisions that will look great as soon as they're planted.
Cut a circle around the hosta clump with a spade, beginning about 6 inches from the edge of the crown. Pry the clump loose and up out of the ground.
Wash all of the soil from the roots in a bucket of water, or use the garden hose sprayer. Don't worry too much about damaging the roots, because they're very tough and can tolerate quite a bit of abuse. The more soil you're able to remove, the easier it will be for you to see where to make your cuts. Try to keep as many healthy roots and young leaves intact as possible.
Cut or break away some of the older foliage from the clump. Remove any stems that appear weak or that have been damaged.
Spread roots apart and out of the way so that you can see the hosta's entire crown. Use a sharp knife to cleanly cut the crown in half. In exceptionally large specimens, a hacksaw may be necessary.
Divide clumps further as desired. Any division with at least two to three healthy leaves and plenty of roots will grow just fine.
Cover the roots of hosta divisions with moist peat moss or garden soil and set them in deep shade if you can't replant them immediately. It's important that the roots not be allowed to dry out. Alternately, immerse roots in a bucket of water treated with liquid fertilizer for up to 24 hours. Hostas soaked in water for an extended period will begin to rot.
Plant the newly divided hosta at the same depth that it occupied before you dug it up. Combine backfill with equal parts peat moss and organic compost. Hostas benefit greatly from rich, fertile growing media.