There are only two means hostas can be propagated: using seeds collected from mature plants and division. Division is the easiest and fastest way of multiplying your hosta collection without buying more hosta plants.
Besides acquiring new plants, division should be done with older plants when no shoots are growing from the center of the mature clump and this bare area detracts from the appearance of the plant. Division of the clump will improve the plant's appearance.
Spring is the easiest time to divide your hostas. As soon as they begin to appear above the soil surface or mulch, you can divide them. There are several techniques that can be used to make this division.
Though spring division is easier, summer division is preferred and can be done in August, at least 30 days before the first fall frost date. Warm soil and higher humidity at this time promotes better root growth, and plants may put on a little growth spurt at this time.
The simplest method for hosta division is slicing. This is just removing a wedge of the plant and replanting that wedge in a new location without removing the mother plant. If you can image your hosta clump as a pie, you'll be slicing out a 1/4 piece of the pie without removing the entire pie. Once the slice is removed, fill the hole with additional topsoil or garden compost. The mother plant will then grow back into this filled area in a few weeks.
If you want a number of new plants, cut larger slices (as much as 1/2 of the original plant) and cut those slices in half after it has been removed from the soil. Always include part of the central core in each slice of the hosta.
This method requires a little more work and it takes longer for the mother plant to recover. With a spade, remove the entire hosta plant as one large clump. This clump is then cut in half and both halves are then replanted. This method is best done in the spring just as the tips are beginning to show. Dividing can also be done in early fall.
Lift the entire hosta clump and wash the soil from the roots, if possible, to make it easier to see where to cut to divide the clump. Cut with a sharp knife to make the divisions. Once the entire clump is out of the soil and split into 2 — 3 parts, cut each of these clumps into smaller clumps, always including part of the center in each clump.
In early summer before the heat of summer has arrive, cut the leaves off of the hosta so about 1" of the stem remains above ground. This heavy pruning causes the latent buds of the hosta to break into growth, thereby producing more growing tips. Then use any of the above techniques to divide the plant later in the fall.
This method is similar to the mowing technique for increasing the number of buds that will be divided later in the year. This method might be a little difficult for most gardeners to do since you have to do it in the spring when the hostas are first emerging from the soil. Usually at this time of the year, seeing anything growing up through the soil is cause for excitement after the long winter and so it's hard to smash this new growth, but that's exactly what you have to do.
As soon as the plant begins to emerge, take your foot and grind it down on the young shoots. This method will work just as good as the mowing method of producing more growing tips for later division.