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If you drink lots of soda or bottled water, this option may be perfect for you. Collect the bottles or cans until you have enough to fill about 1/3 to half of your tall planter. Leave the caps on the plastic bottles, and avoid crushing the aluminum cans too much if you’re using those. If you use this option, you’ll also want to have a layer of newspaper, landscaping fabric, or cheesecloth available to lay down on top of your bottles or cans before adding your soil. Choose a pot that fits easily into the center of your tall planter. Be sure that this pot has its own drainage holes, or add some to it by drilling them in.  If your large planter has drainage holes along the bottom edges, avoid blocking them with the top edges of your smaller pot. You’ll need to be sure that water can still drain through the bottom of your planter. If your smaller pot is significantly smaller than your larger planter, you’ll want to hold it in place with another material like clay pebbles or wood chips. This will also help to lessen the amount of soil you’ll need when filling your planter. Scrap 4x4 or 6x6 pieces of wood also work well for filling the bottom of your tall planter. Choose a piece that easily stands up in the center of the bottom of your planter. Using a saw, cut the wood so that it’s about half the height of your planter.  Avoid this option if the drainage holes of your tall planter are in the middle of your planter’s bottom. The wood will cover the holes and not allow your soil to drain properly. You can use wood chips, clay pebbles, or soil to hold the wood beam in place in the center of your planter. This option is a bit heavier, but still works well to defray costs when buying your topsoil. Collect enough rocks or broken pots to fill about 1/3 to half of your tall planter.  This option works well if your tall planter becomes narrower at the bottom and you’re concerned about it tipping over from the weight of your soil and plant at the top. Be sure that your rocks are not covering any drainage holes in the bottom of your planter. Collect enough pinecones to fill 1/3 to half of your tall planter, or purchase wood chip mulch at your local hardware store. If you use this option, you’ll want to be sure to check the filler each year when you change soil to make sure the wood hasn’t started rotting. If you have some packaging peanuts lying around, test them out to make sure they don’t dissolve in water. Place some in a bowl of water and check to see if they shrink or dissolve. If they don’t, you can use these to fill 1/3 to half of your tall planter for a planter that’s nice and light.  Use mesh onion bags to hold the packing peanuts in place and avoid making a mess when you replace your plant’s topsoil each year. You can also try cutting up larger pieces of Styrofoam packaging and filling your planter with these pieces. Plastic is light and lasts a long time, so any plastic items that fit into the bottom of your planter are a nice choice. Use intact Easter eggs, keep lids on milk jugs, and keep the lids on containers if you’re using those. This will keep most of the bottom of your planter filled with air, and allow for good drainage from the soil above. If you’re using jugs or containers, be sure not to block any of the drainage holes in the bottom of your planter. Space the jugs or containers out so they’re not blocking the holes.

Summary:
Use collected soda bottles or cans for a low-cost, long-lasting option. Try a smaller inverted pot if you have an extra one. Stand a tall, rectangular wood beam into your planter for a simple choice. Use rocks or broken crockery for a sturdy option. Choose wood chip mulch or pinecones as a lightweight choice. Use Styrofoam packing peanuts if you have them. Try plastic Easter eggs, milk jugs, or containers as a long-lasting choice.