The natural-looking border uses rocks, wood, and plants to make it seem like it was naturally there. You can use natural borders in any style you like. Make an informal border with rocks or wood, and allow groundcover to spill over as if it has been there forever. it's a very appealing look and I plan on reworking one of my edges to look that way. You can also use natural elements in a more architectural, linear way. It's pleasing to the eye because the natural elements create a soft organic flow and laying them out in clean lines creates a modern, organic design quality to your garden.
You can find these natural elements for free everywhere--you just have to be on the lookout at all times. When big storms roll through where you live, you are actually helping when you collect wood for your garden borders. All you have to do is haul it away and chop it up, and BAM! instant border material.
Rocks, I've found, are trickier to find for free. If you live near rivers or lakes, there are probably lots of rocks. You can ask construction companies if they have discarded rock or even broken concrete that you can use. Be careful not to take anything off private property without asking! Ask around; friends and neighbors may have rocks they can give you. With a bit of patience you will have enough rocks to do what ever your heart desires. Making mock rock out of hypertufa is also an option. The how-to's on that can be found in the hypertufa forum.
Using art in borders can be fun, useful, and good for the environment. Gardeners have been doing that for ages. You can be as wild or as mild as you please. Tires, for example, are a classic reusable object for bordering and planting. Doing an internet search of "tires in gardening" will pull up all kinds of great ideas. You can just cut them and use them like the black roll edging we all know. Just call around to mechanic or tire shops and you may get some freebies.
I used plates and cinderblocks to border my lotus bog. The blocks were here when we moved in, and they kept getting moved around, looking for a home. Do you have things laying around needing a purpose? I'm not referring to your teenage children, spouse or the dog, either. I planted the holes with sedum and hens and chicks in the hopes that it would eventually look like a small old concrete wall that has become overgrown.
What happens to bowling balls when they get worn out? They, too can go in the garden! Need ideas? Try painting them to look like bugs. Or you can mosaic them or leave them as they are. Place them on top of the ground or sink them for semi circles. You can get crazy and put things in the holes for a dramatic effect.
I know you have old flower pots laying around. Or at least you know someone who does. Paint them, mosaic them or not. Sink them into the ground a bit and plant with low growers or fill with something for a colorful border. You will also reduce clutter.
If you like an organic feel but you want to repurpose the man made items, here are some ideas. Incorporate a natural feel to unnatural elements by covering them mosaic-style with small pebbles, shells, twigs or any natural element that will stick. Natural paint colors or scrap earth tone tiles will also do the trick. Calling around to home improvement stores to see if they will sell the broken tile very cheap is a great idea--I've had great success with that approach. Save your money for the plants, and search thrift stores, yard sales, even junkyards for your edging. Reuse, be creative, and have fun!
Some things you can use for edging:
any type of old lumber
broken shovels ....
any one else have an idea?