Why divide ?
For perennials, there are two ways of reproduction: sexual (seeds) and asexual (divisions, cuttings, ...)
Most perennials in culture are cultivars, crosses of wild species or other cultivars. Most of these cultivars do not come true from seed. The only way to propagate them is by taking parts of a plant and growing on these divisions to become new plants. For many cultivars this is the only way to propagate them.
For hostas, dividing is extremely import, because not only cultivars do not come true from seeds, the same applies to almost all species when grown in cultivation.
Maintaining instable plants (streakers)
Streaked hostas tend to be genetically very instable. After a long or a short period, most streakers settle down in a more stable form, with a clearly distinguished edge and center in different colors.. If, however, you're into the hybridizing of new two- or multicolored cultivars, you want to keep the pod parents streaked. This can often be achieved by dividing the pod plant on a regular base.
Maintaining a juvenile form of a plant.
Some hostas, like H. 'Ginko Craig', have a totally different look as a juvenile and a mature plant. If you want to maintain or regain the juvenile form, you'll have to divide the plant regularly.
However, it's not necessary to divide hostas, as is sometimes claimed. If a plant is in a good sport, with sufficient nutrients and water, and is left undisturbed, only them will it grow to a mature plant in all its glory.
Rejuvenating a plant
Sometimes older plants get centered out: the center of a clump dies back. The remaining healthy parts are divided into healthy divisions and planted again.
When to divide ?
To a plant, dividing is a traumatic experience. To keep the consequences of this trauma as limited as possible, it's best to divide it at a time when it has the best chances of recovery and regrowth is at its peak.
As I have limited experience in dividing (almost none of my hostas were divided before 2007), I asked the expert's advice. Danny van Eechaute of the "Hostacollectie" in Deinze, Belgium, has an impressive collection of about 2500 different species and cultivars. Because all the plants are kept on a relatively small plot of land, a lot of divisions have to be made every year.
According to Danny, the best time for dividing hostas is June - July. The first flush of leaves has been formed and the plant is getting ready to form new roots, thus giving a good regrowth.
If you have a large number of plants however, it's not possible to divide all the plants in a 2 month period. You can divide hostas safely starting in spring, after the last frosts, until autumn, when the leaves start to die back.
How to divide
What do you need ?
- garden plants in the garden: a spade, a sharp knife, compost to improve the soil structure, nutrients and, water
- container plants: a sharp knife, potting soil, nutrients and water
- always: disinfectant to prevent the spreading of viruses and nematodes.
- remove the plant from the soil or the container
- remove part of the old soil
- Determine in how many pieces you're going to divide. Based
on this number and the size of the divisions, prepare the
planting holes or containers.
A planting hole doesn't need to be very deep (20 à 40 cm, depending on the size of the adult plant is adequate). The width equals the expected diameter of the mature plant. Improve the soil structure with compost, humus or any other substance suitable for soil improvement at you disposal and a nutrients
I like my containers very large. As for the potting soil, its must retain water very well, without remaining very wet and should be easy to get moist again when it has dried. Add the necessary nutrients.
- cut the plant into as many pieces as you like with a clean, sharp knife.
- remove dead roots or parts
- plant out the divisions. Fill up the pot or the hole partially with the improved soil and make a flat cone in the middle. Spread out the roots over this cone. The place where the leaves meet the crown should be level with or just below the surface. A little deeper is acceptable, not deep enough is wrong
- water in well, and keep doing this for the next couple of weeks, until the plants shows signs of good regrowth.
Prevent HVX, nematodes and other contaminations from spreading!!
When dividing or replanting, always take strict safety precautions. Always keep in mind that most plagues, like HVX, spread through plant sap. Nematodes spread through moisture.
- whenever you use a tool, like a knife, scissors, a spade
with hostas, make sure they are free of the virus or plague:
- first wash it up in a solution with regular household cleaner;
- dry it with clean paper towels;
- let it soak in a disinfectant solution for at least 10 minutes (bleach or any other household disinfectant like Lysol, Dettol, ...);
- dry again with clean paper towels;
- let it dry until there is absolutely no humidity left.
- destroy remaining plant parts; don't mix them with your other compost;
- be extra careful when you suspect an infection
- destroy plants that are HVX infected;
- quarantine nematode suspected plants;
- don't plant hostas for a longer period of time in or near a spot where an infected hosta has been.