I'm making some large containers (SAY: 12" deep x 18" wide x 10 feet long). Several of these will be on the second floor, so I want to minimize the weight by not filling them full of potting soil.

My idea is to fill them about half full of plastic packaging peanuts (instead of gravel), then cover the peanuts with some kind of plastic mesh. The potting soil would be above the plastic mesh. The peanuts are to provide drainage and the mesh is to prevent the potting soil from filtering down into the peanuts. (I'm thinking the mesh has maybe 1/8" openings).

The peanuts can be pushed aside for deep-rooted plants, and piled up for shallow-rooted plants (again to minimize weight)

1. Does this sound like a good idea?
2. Does anyone know a source for cheap plastic mesh?

to ATP
The thread "What filler at the bottom of large pots do you use?" in Containers forum

Might be some ideas in this thread. I hate those packing peanuts.

There's a couple of kinds of packing peanuts, I only use the hard ones in pots, they kind of 'crack' when broken.

Hi & welcome! Adding copious amounts of perlite to potting soil can give roots room to roam, while reducing weight by volume. I would expect better growth from that than by simply using a smaller volume of soil which would be more likely to inhibit root growth.

Landscape fabric would serve your purpose, available in rolls you cut-to-size needed.

You could use empty water/soda bottles to occupy space w/o adding weight. Empty, upside-down pots can be buried, then easily removed if you want more root-space later.

You could sit pots in the planters, surrounded with a decorative moss, or something like pine cones, if a "filler" is needed. This would allow easy replacement as annuals finish up, and to have a display of plants with varying moisture needs.

Assuming your planter has a hole in the bottom? Excess water that can't drain out could definitely build up excess weight.

Don't forget to check below before you water, or at least holler, "umbrellas up!" LOL!

Bobdoss.
Sounds like you have some great ideas from several people. Let's see what you decide on.
I look forward to seeing photos of your completed project.

I usually throw in some empty plastic bottles of varying sizes to about a third or half the pot size. The soil filters itself around them giving something for the roots to move through.

I'm like Anne...I use the water bottles also, but I use more of the medium sized ones so that I can place them closer together. I use window screening (which comes pre-cut and on cardboard rolls) to place between the bottles and soil. I used to use peanuts but they are a mess to clean up if you want to make any changes and I used to use the very hard Styrofoam blocks that came with TV's but they are hard to cut to fit into round pots.
Oh...and Bobdoss!

Some of the modern "environmentally conscious" packing peanuts are made from cornstarch and will dissolve in water. Either way, plastic or cornstarch, I would not recommend using those. The alternate ideas are all good; vermiculite, pine cones, empty plastic bottles, upended flower pots. Around here we pick up the beer cans from the roadside while walking the dogs and place them up-side-down in the larger plants, covered with landscape fabric.

Great way to get exercise and save on potting soil Greene.