Elder is a tree. You've probably got Sambucus canadensis, which is smaller than S. nigra, which I have. Both like much the same growing conditions.

My tree only gets water when it rains, and since we're in the worst drought in recorded history, that hasn't been very often for the last few years! I'm not getting much fruit, though I get plenty of flowers.

Yes, it does do best in fairly moist soil, but if you give it a drink (or get rain) from time to time, it'll be just fine. It might become a weed, actually, if you water it too often! Those roots can spread far and wide, and each one of them can become a sucker (new plant). I find that nothing much will grow within reach of those roots - they have a growth inhibiting effect on other plants. Mine's in full sun - remember, that's fierce, subtropical sun!

Rock must go. Roots of nothing will grow through it, and elder likes a LOT of room to spread. When you want to dig out those suckers, you don't want to be digging through rock, either. Rocks don't provide drainage - a good friable soil with plenty of organic matter in it does that, or planting a plant on top of a mound or hill. Or both.

My plant takes up a fair bit of space. It gets to 10 metres tall. That's taller than the top part of the roof on my house. I have to keep chopping it right back to maintain it to a reasonable size. Every time I do it, it grows more and more new stems. One day, before long, the entire thing will have to be dug out and replaced with a baby, because it definitely will become unmanageable.

Your plant (if S. canadensis) will get to about 4-6 metres tall. But those roots will take over if you're not careful!

Every time I've ever seen a large stand of Elderberries in any State we've ever lived they've always been on the up-side of a down-hill slope to a ravine in a more rainy area.... so they get the moisture but have it drain away quickly... so only dampish soil. Even in California's more arid climate you'll find them growing along the roadsides in the mountains where it's still more dry, but where it does rain off and on all summer.

Hi, Ifound a great site about Elderberries -

"American elderberry is a thicket-forming shrub with large flat-topped clusters of small, fragrant white flowers in spring. The flowers are a good source of nectar for small insects. Big clusters of dark purple to black, berry-like fruits follow in late summer to fall. The fruit is an outstanding source of food for birds and other wildlife. The old stems provide over-wintering sites for insects."
woodpeckers then forage in the clumps for the insects.

http://www.abnativeplants.com/_ccLib/image/plants/DETA-28.jpg

We used to have lots of Elderberry bushes on our property and really didn't do anything for them to produce. We only cut back to maintain the form and help it produce. They make great jelly and syrup for pancakes.

Have fun

Here is a link that might be useful: