Hostas are greedy feeders, that will be grateful for a rich, well drained soil with loads of organic matter.
It is good practice to have your soil analyzed before you start adding all kinds of fertilizer and organic substances. Based on the results of the tests, you will get professional advice on how to change the soil into a hosta paradise.
If, like the Hosta Mill, your garden soil is far from ideal for growing hostas, you'll get the advise to improve the soil structure by adding organic substances, like well rotted stable manure, compost, etc. The real black gold is leaf mould (well composted leaves), if you can lay your hands on it. Besides that, you'll have to add fertilizer on a regular base. More on this topic under The soil.
Here's a question of mine: in US sites, I frequently run into "composted bark" as a part of the growing mixture for hostas. I've never seen it in Europe so for. Can anyone inform me about the benefits of composted bark in hosta growing? Which bark do I use ?
The ideal hosta soil is well drained, and slightly acidic (pH 6,5), although a 7,5 pH is still pretty good.
Hostas have a rather shallow root system, that can spread far. So, when you are planting in planting holes, they only have to 12 to 16 inches deep (it's never wrong to dig deeper, especially when dealing with extra large plants), but at least 1,5 as wide as the expected diameter of the mature plant.
Hostas in pots or containers
A container grown hosta is no different from a hosta growing in the garden.
If grown under ideal conditions, most hostas in a container will
perform (almost) equally well as those grown in the garden, some even
better (the minis). You have to bear in mind that the quality of
the growing mixture in a container and the level of nutriments can
deteriorate rapidly, since hostas are such gross feeders. A
container grown hosta in fact needs a "planting hole" that is as large
as that of a hosta grown in the garden (which can leave you with some
huge container requirements).
Large hostas need a large container !!.
First you have to prepare the planting hole (or the container, that you fill partially with potting soil). Get the plant out of its container or out of the soil. It's best to remove at least part of the old potting. Spread out the roots horizontally in the hole. If a container grown plant has made a root ball, container shaped, you have to untwine it (partially). This will stimulate the plant to form a natural, more horizontal root system, and it will grow better.
How deep should you plant ? An easy rule: as deep as it was. The crown, the part where the leaves grow from, will be just below the ground surface.
Mail order plants arrive bare root. Soak them for at least 1/2 hour in lukewarm water.
After planting I dust the surface with a bit of bone meal (a trick Danny Van Eechaute taught me. This is very beneficial for root development, a crucial factor for a recently replanted hosta.