I think a couple of inches of yard waste compost spread out about at least three or four feet in every direction each year works well. The best thing is to have as good and rich (with nutrients) and deep and wide and loose (not compacted) as you can.

What growing region?

It depends on what you mean by shallow topsoil. If you mean shallow "good soil" with heavy clay or sand underneath it, it really pays to enhance your soil ahead of time before you put a rose in to make it happier, as Sam says. You can simply dump organics on top of the soil and the worms will eventually mix it in for you, but that isn't a short-term solution if there isn't much soil to work with.

If the soil is shallow because there's rocks or tree roots or concrete or something else inorganic below it, that's a problem. Climbers need to have good root systems to support all that top growth, and while some of the roots grow sideways, the anchor roots tend to run deep in at least a few places. Also, if the soil is only a few inches deep, there isn't enough nutrition to support a rose of any sort.

If you fill us in on details as well as approximately where you are, we can help more.
Cynthia

Hi, I'm outside Boston-- Zone 6A.

In one spot I'm thinking about, the topsoil is maybe 4" deep with sandy soil under it.

How deep do you think I should make a soil 'pit' for it?

Do you think a climbing rose can be near forsythia bushes and do well?

Thanks!

Roses are heavy feeders . They like it rich in organics. Root competition can be a problem. The forsithia may win the battle. The forsithia roots will sneak into the the nice rose soil. A rhisome barrier would do the trick. A big piece of metal like an old street sign in the ground to block the roots. But that is a lot of work.

A normal topsoil layer is around 10" deep and the fine roots of roses mainly develop between 2" and 10" even if the good soil goes deeper. So I normally recommend that poor soil need be amended only about that deep. But in most gardens the subsoil is clay and heavier than the topsoil. It serves as a reservoir for water. Sand as subsoil is more of a problem because it retains hardly any water, and, paradoxically, a heavier topsoil will not drain into sand below. It creates what is called a "hanging water table" if you want to learn more about it.

I would dig the area around 18" deep and incorporate a 2" layer of clay (use clean, plain kitty litter) as well as several inches of compost, including some manure. Look up the "double digging" technique that is used to add amendments at depth.