The whole hosta garden at the Dallas Arboretum is planted in raised beds.

bk

Ok, more detail. The "rooty" part won't change. The tree roots will grow into your raised beds, as they will have better soil and more water.

The Dallas Arboretum has planted in the shade of crape myrtles. I don't have crape myrtles, myself, so do not know about their root habits. We have huge numbers of crape myrtles in the area, but I've never heard anyone complain about the roots. Their beds are drip irrigated.

Hosta do not have tap roots. They have spreading roots like an oak tree.

I would go about 8" if you are doing it this year. If this is a future project, I might consider "lasagna" bed preparation to break up the hard pan you have there and then add your soil when you get ready to plant.

This is a really busy time on the forum. Hopefully you can get more attention than just from me, hopefully someone who knows more about pines. If you will reply to this post, it will kick your post back up to the top, where it might get some knowledgeable person's attention.

I live in Texas, so may have totally different conditions. That's how I would do it if I were building a raised bed here. I would also add peat because we have alkaline soil. I would not use peat in any other circumstances.

bk

I can't speak to your specific growing conditions but in my own situation, I ignored tree roots when planting hosta. As I dug planting holes beneath a mature crabapple tree, I simply yanked (or, in some cases with thicker roots, cut) roots as I encountered them. As an "experienced" garden neighbor pointed out to me, the tree doesn't survive or depend on that one root alone.

Can hostas be planted in raised beds; do hostas have long tap roots; how deep should the soil be, etc.

Hostas likely would adapt well to raised beds although I plant my own at or slightly higher than the same level as surrounding lawn.

Hostas do not have long tap roots.

I can't speak from experience as to how deep the soil should be in a raised bed but when I've dug up & divided mature hostas, the root ball has been a minimum of 10 inches in diameter & depth.

Crabapple corner many years ago when first planted. The bed is full now & plants are mature.
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To the right, full shade bed in early spring. Hostas: Queen of the Seas, Blueberry Muffin, Touch of Class, Krossa Regal, Regal Splendor, Dream Weaver (x4), Frances Williams, Revolution
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It very much matters what type of tree you are planting under. If they are invasive-rooted trees (trees that keep their roots near the surface), then you are going to have trouble, like Bkay said, with the roots coming into the bed and strangling your hostas. Maples (not Japanese) are one of the worst for that. If they are trees that their roots tend to grow down and not out, you will be fine.

If you are thinking of planting with a raised bed under invasive-rooted trees, then it would have to be pretty deep and you would have to come up with a way to block those roots from coming up into the raised bed. That's a lot of trouble. Maybe look into spin-out bags for your hostas. Those block the roots from coming in and can just be sunk into the ground.

I don't have any trouble at all planting around pine trees (White Pine). I also don't have any trouble around big maples with Sieboldiana Elegans.

I would say that the raised beds could be a height convenient for you, maybe seat height as a convenience and if you select big tough hosta you should have no trouble. Smaller hosta with thin wimpy roots will have a far more difficult time competing and surviving.

People must be tired of seeing this picture....

...but this is my Elegans directly under a maple tree.

Jon

There should be a list of workhorse hosta.

bk

Raised beds will work fine.

The tree roots will invade it in the first year.

You just have to water it frequently.

The cherry tree here and the lindens on the other side of the fence have made this side of my property pretty barren. (Crappy clay soil as well)

Only Sedums thrive here.

The raised portion is 3 yards of triple mix, with some extra bags of manure mixed in. (bed is roughly 17X9)

I put about 4 bags of manure on top in the spring as the starts to "sag" a bit each year (the organics get consumed)

I do a good fertalization in early May. and some good forest mulch (Hemlock is best) every couple of years.

J.