By Bob Axmear
This is my version of dividing hostas. Bill's below is more on the technical aspects of it. I like to divide them early in the year so they recover before they leaf out and/or it gets too hot.
I dig the plants up in early spring before they have a chance to come up, just as they are breaking the ground. I use a stainless steel spade and dig around the plant, these spades are unreal how much better they work than steel spades. They never rust and are always polished, takes most of the work out of it. Then I take the spade and slice under them by repeatedly stabbing, about 4 inches or better horizontally below. This seems to work better than trying to pry them out and breaking the handle. I have broken a dozen shovel handles at least doing it that way.
Then I take a high pressure hose and wash all the soil off, a garden hose with a nozzle on it. This is so you can see the growing points clearly.
I take a butcher knife and cut between the growing points. I also tug them apart as much as I can to minimize the cutting and damage to the roots. Sometimes it is easy to do if they are growing widely apart and sometimes they are close together. Even if they have a very small amount of roots when done you can pot them up and keep them in the shade for a few months, well watering them and they will reroot.
Sorry about the looks of the divisions. We had a cold snap lately and it froze the outer leaves of some.
I try to add 2 to 4 divisions for small to medium plants and sometimes just one for large plants with huge divisions but this is all up to the person doing it.
I pot them at the same depth as they were before, just put them a little under the soil line. I use quart pots for these, they are gone in about a month to 6 weeks at a town garage sale. If longer term I would use a larger pot. They recover nicely in that time as long as they are potted well and are kept very moist. Never let them dry out during this time and I keep them in the shade as well. This is my wife helping me, she is scared to get her hands dirty I guess.
These are from a month later.
Dividing Hostas During the Summer
Dig up the plant with as many roots as possible.
Wash all the soil off the plant so you can clearly see the divisions. This will help you see where to cut or tug the plant apart.
It is up to you to decide how much to divide the plant. If you divide it too much or a division has few roots you can cut off some of the leaves, pot it up, and put it in the shade and water it often and it will reroot. I have had them root with just a few bumps for roots but you really need to limit the leaves in that situation and keep it moist and shaded.
I replant most immediately and water well, I water every few days for the first month or so. This might be excessive but they show little stress if watered repeatedly. If it is very hot in the summer I pot them up and move them into shade and plant them again in the permanent spot in late August in northeast Iowa, might be later farther south. Never take a hosta that is in the deep shade and try to move it into more sun during the heat of summer, sunburning can really kill hostas faster than about anything.
This is a good question, but not one with an easy answer. When you divide them is one of several factors that need to be considered. You have to look at the whole picture to really decide what to do. I've been trying for a long time to get a grip on this one, and I think I'm starting to understand it. The main factors to be considered are:
1. Time of division.
This means relative to their growth cycle, not by calendar date. A hosta sends up new top growth from stored energy in the rhizome. Once that top growth is made, it begins making new roots to support that top growth and more if the growing conditions are good. Then it settles down into storing energy for the next cycle. Different species offer some variations, like multiple cycles in a season, or continuing top growth (plantagenia), but there is a basic cycle that you can see happening.
2. Environment adjustment.
Hostas adapt to their environment. We've all seen the way they make different leaves in sun vs. heavy shade. More important to how well they grow is how the roots adapt. The roots are produced in the form that is best adapted to the soil it finds itself in. Light potting soil results in big networks of thin roots while heavy clay soils result in a few thick ropy roots. The wrong roots in the wrong soil will mean poor growth, because a hosta will react to not getting enough water by getting smaller in an effort to survive. If a hosta has the wrong roots - like pot roots in the ground - it will not be able to get enough water no matter how much you water it. The same applies to a divided plant which has lost roots through injury.
More stress means less growth. Hostas grow fastest if their growth is never interrupted. If they dry out they slow or stop their growth. Once this happens you will get a smaller plant. Drying out is not necessarily the result of dry soil. The leaves lose a lot of water through evaporation, and if the roots can't replace that water fast enough the plant isn't getting enough water to keep growing at its maximum speed.
4. Your ability to care for them.
Once you embark on a course of dividing and growing them on, you need to stick with it to keep growth going at the best speed. If you have plants that need a constant supply of water and you go on vacation and they dry out, the growth gets interrupted. If they need a lot of shade and a tree gets knocked over in a storm they will lose their leaves and slow their growth.
The factors are all tied together and you can't ignore any of them if you want maximum growth. Consider all of them before deciding what to do. Hostas can take a lot of abuse and survive, but if you want maximum growth you need to go the other direction and keep them in intensive care when they are under a lot of stress from being cut or being transplanted into very different soils.
For example, if you want to cut a plant that was in the ground for a season, remember that it is losing a lot of its ability to take up water because the root system has been severely injured. If the leaves are not fully developed, cutting it will stop the development at a smaller size. If it has fully developed leaves it will be vulnerable to drying out without extra care as the leaves lose more water than the roots take up. If you cut it early (after it made full top growth but before it makes new roots) it will grow new roots to match its top growth. If you kept it from getting too much sun the leaves should all last until the new roots can support them and the plant will start new growth from there. On the other hand, if it got so much sun that some of the leaves died because they dried out, it will produce only enough roots to keep that smaller bunch of leaves going and end up a smaller plant. If you cut that plant in the Fall, it will only send up top growth the next season to match what the roots can support, then resume growing.
Transplanting from potting soil to the ground follows pretty much the same pattern. Although the plant keeps its roots, the result is the same as injuring the roots on a plant that was grown in the same soil. Because the roots are adapted to the soil, they don't work well in different soils. Having all the wrong roots is not really any different than having only a few of the right ones. Either way the plant can't get enough water to the leaves to keep them from drying out.
I guess what this means is there isn't a simple answer. Hostas are so tough that we tend to look more at how much we can get away with abusing and neglecting them rather that at how we can get them to grow best. If you think about how much stress they are under from cutting or transplanting them and try to relieve that stress as much as possible, you are on the road to growing them as quickly as possible.