How deep is it and do you have any pets or children?
What is the underlying ground like: clay, sand, ??

It's about 2 feet deep in the center, shallower as it slopes up to the lip of the pond. No children, but we do plan to get a dog in the future.

The surrounding ground is just regular soil*, not sandy or crumbly. The woman who lived there did a lot of gardening before she passed away, so the surrounding area probably had flowers.

*Sorry, I know that's not very descriptive! I'm completely new at this so I don't know the terms yet!

I think your best bet is to fill the pool with the roundish river stones that you can buy at a landscaping materials store. Because you want to divert your rain gutter downspout to it, it would be hard to recommend using it as a bog especially since it is in a shady location. Bogs really need direct sun for several hours each day. If it retains water for any length of time you may want to try a couple of marginal aquatic plants for aceent with a large boulder in the pool but to one side of it. Check locally with a nursery for what is available and that flowers. If you go this route I would just take a potted plant and sit it in the pool on top of the stones but with the stones about a foot below the rim of the pool and sit the boulder and your potted plants on top of the round stones and then fill in with pea gravel. Perhaps topping off with only a few flattish or irregular stones to add texture to the scene.
Visualize it first and then paint your living picture!

Thanks for the advice, terrestrial man. The reason I wanted to divert the downspout (and how I found out about this type of garden in the first place!) is because I read a cool article about making a "downspout bog garden". I thought it would be a good use of water that would otherwise go to waste (we are putting rain barrels around the house, but not the garage) and require little maintenance.

It may be that it gets more sun that I think, we aren't actually in the house yet so I'm only guessing based on the trees in that section of the yard. It seems pretty shady, though. Why do bog gardens need several hours of full sun? So they don't become too swampy? Or because the type of plants that grow in one need it?

Cement is made w/ lime. It can leach and make your soil basic. Your adding peat which is slightly acidic, the cement will probably buffer that acidity so you might end up w/ neutral ph, hard to know. Normally not a big deal unless you want to plant acid loving plants. Bog plants like carnivores (Sarracenia) and many orchids like acidic soil, full sun.

A "true" bog ecosystem, as found in the "wild", has acidic nutrient poor soil and full sun.

As T-man says, sounds like a good place to grow marginals as long as they get enough sun. One good way to go about it is to figure out what conditions your site provides... wetness, amount of sun, soil type.... then find a plant community/related plants that would thrive under those conditions.

Drats, sounds like I'm going to end up with more of a pond than a bog. May be my contrary nature, but now that I know it's not the best spot for one, I want it more! The shade issue I can't do much about (other than chopping down my tree!) but maybe I can find natural additives for the peat/soil to make it more acidic.

I'll search for plants that are native to my area that like shade and wet conditions and see what I can put together. As I said, the area isn't naturally wet, but that cement hole is just sitting there begging to be used for something!

I recommend equisetum hyemale, the Horsetail rush.
It'll grow just fine, and you can keep it contained (in a pot) if you're
worried about its invasive capacities. I have equisetum in-ground in a
container, and in-yard in a container. The in-ground container is
markedly larger - not only are the spears twice the diameter, but the
spears shot up nearly three weeks before those in the container did.

Josh

Josh, I had to google that plant, never heard of it before. It's pretty neat looking! When you say "in-ground" do you mean it's in a pot then sunk into the bog?

Yes, indeed, in a container buried in the bog. Equisetum is often sold standing in water, in a container with very little drainage. It's a good way to keep these things under control. And, keeping them in containers encourages a tall, thick, striking growth of spears. Sometimes I loop some twine around my group to coax them upright. You can divide the horsetail, too, and stage it throughout your bog or garden. I have it growing amongst ferns.

Josh

Excellent, thanks for the suggestion!

I have my E. hyemale planted in a sealed bottom half barrel container after it escaped under a deep barrier. After two years of serious digging and RoundUp, I finally got rid of all of it except for what I keep totally contained.

I have a fully containerized equisetum, but many of the new spears seem
to have spore cones this year...I'm wondering how well they might establish.
I might have to isolate the container on a deck or slab...

Josh