What are some good flowers that have shallow roots systems?

I have small backyard I share with my nature-loving daughter. There are old rusty pipes that are too heavy and too deep to remove, so I covered it with a large pile of brush and cane. It is along a fence in a grove of young trees so underneath the pile is a mass of tree roots. I'm planning to put a mound of soil over it about 5 or 6" deep to hide it. My daughter suggested it would be a nice place for a fairy garden but I wonder, if I plant perennials or flowers that will attract butterflies, will they thrive in such conditions? What plants could I use?

It's a fairly sunny area and I live in zone 5 in New Brunswick, Canada. So, a very short summer and very cold long winters.

petunias, if you have afance try some morning glories, some impatients.. this also all depends on what kind of sunlight this area gets....inpatients, lillies of the valley, mini rose bushes, are some

Rhododendrons and azaleas both have very shallow root systems. most perennials also have relatively shallow root systems. Bulbs are great, their roots never run very deep. A butterfly weed is a perennial that attracts tons of butterflies.
It would work better, i think to simply cover the pipes with soil up to the height you want, and then planting. The plants will be able to find the soil beneath the pipes. But if there is a loose pile of brush between their 6 inches of soil and the ground, the roots would be doomd, as very few plants have root systems that shallow. this would also allow you to plant a wider variety of plants, maybe even a butterfly bush.

Most any annual has shallow roots, you said young trees, so is it dappled sun & shade or always sunny, half-day sunny? This is the only other element that needs to be taken into consideration. Cosmos is easy from seed with many types of flower shapes & delicate fern-like leaves, cleome is somewhat taller but may also work & is easy to grow from seed. Chamomile can grow under similar conditions, evening primrose grows best where it's hot & poor quality soil & reseeds itself, 2 types, 1 yellow, 1 light pink. Zinnias & marigolds, allyssum-in white, purple, or pink & honey-scented, lobelia is a dark blue usually, so is heliotrope but the latter is scented. English daisies-the kind found in some lawns are usually treated as annuals here in the U.S. could also be used.