I'll transfer the pics over here for you.

Those appear to be mature Willows to me. You can see in the first picture that the roots are visible at the surface level and that the grass is having a difficult time growing in that area. Planting small unestablished Hosta in that area is going to be very difficult.

You can't just raise the soil level. Firstly, you need to continue to be able to see the "flare" of the trunk at the soil level. If you cover that part of the trunk in soil you will eventually kill those trees. Also any amended soil you add will just attract the willow roots. They go for the good stuff. BTW, the moss merely means that your pH is low.

If this were my yard, here is what I would do.
1. Take a close look at the shade/sun situation with these trees.
2. Reshape the bed making the area that gets morning sun/afternoon shade larger than the side that gets the opposite.
3. When planting closer to the trunks of the trees use either pots above ground, or root control grow bags. Link below.
4. Amend the soil in the areas I am going to plant in the ground.
5. Use the biggest Hostas I can find from Hosta specialty nurseries.
6. Water, water, water. Water by sprinkler over a large area that includes areas beyond the garden, so that the willows can get what they need without strangling your Hostas to get it.
7. Put down a thick mulch of wood chips over even a larger area than the garden. The grass is competing with the Willows for moisture also. Do you see in that pic where the grass is greener? That's where you might be able to plant in the ground.

What you are planning to do, can be done, but it's difficult. In the battle of the roots, the Willows have an enormous advantage because they are so well established, while the Hostas will be just babies. It will take lots of care and water to achieve what you want with the garden and not damage/kill those trees.

Steve

Here is a link that might be useful: Planting using spin out bags

Water extremely heavily. You will be better off watering the entire yard heavily and not just the garden bed as the tree roots will go to where the water is. Willow roots are notorious for seeking out water sources such as sewer lines and septic tanks. My Maple tree roots are the same way.

Hosta can be grown under the trees but it will take extra effort on your part with the extra watering. It is the same with fertilizer, fertilize everywhere. You want to keep the tree roots spread out as much as possible and not just concentrating their growth in your garden bed where all the good food and water is.

I am not sure how the Willows will react to having their roots chopped up when planting things. My huge Maples don't even flinch when I do it. It takes a hatchet and pruners to plant anything in my yard due to the thick mat of surface tree roots.

Hi Gabby,

We made a new bed in the front around our tree.

The only regrets I have is that I should have made it bigger.

Steve has given great advice. I use pots around the tree trunks too. I have solar lighting and gnomes that I set out througout the bed, but we haven't had time to get them out of the shed yet.

This section get the morning sun, some of the hostas are barely above the ground:

Gesila

Willows might respond by sending up suckers where the roots are cut or injured. Some trees do that, elms for example, locusts for another, also hackberry. I don't have a willow so I can't testify for sure tho.

Sandy

Thank you all so very much. This is so helpful!! The trees are in fact Golden Willow, good guess;)

We are just moving into Spring here in Idaho so the grass is not yet as green as it will get. I have underground sprinklers, so I can water this area as heavily as needed.

I will take all your advice, and see how it works out. I actually have an area in back where I have been growing my hosta for two or three years, so I will be adding many established larger hosta to the area along with new. I have a great nursery here that carries very nice mature hosta, so I can focus on those as well.

Thank you all so much. My love of hosta is a recent aquisition, and it has been so much fun!! Now just have to find names for the first 20 or so I bought not realizing I would be much happier knowing their names:)

Thanks again,

G

Gabby -- I think you just hit the nail on the head as to one of the reasons hostas are so easy to love -- they all have names we can pronounce! I walk about my garden every day and identify them -- good mental exercise. I suppose my other perennials can be similarly ID'd, but I have no idea what cultivars they are, and I don't 'talk to them'!
Jan