Mulch will be almost as heavy. Seriously, you can use styrofoam peanuts. Just make sure your soil really is deep enough for your plants.

I just purchased some very large pots also. I used 1 1/2 bags of Walmart's cheapest soil $3.84 for 50 pounds then topped it with Miracle grow mix. I was doing 2 pots. I pondered what I was going to do also and decided against bark or mulch, they will eventually breakdown, but can also bring in termites, roaches, etc. Cheap soil will probably cost the same as the bark or mulch would. I will be watching for comments as I have 4 other pots to use in my garden for vegetables.

First check to see if the urn has drain holes. If not make some with a drill or hot ice pick. (I've done that with plastic containers, guessing it would be okay with fiberglass, but check it out with someone first.)

Throw some empty plastic containers, such as milk bottles in to at last half full, maybe more, depending on what you are planting.

Miracle Grow Potting soil works for me. Don't forget the proper fertilizer.

Sue

Call a landscaper and find out if they have any mulch that can be used for outdoor pots. I hired a gardener to turn my island flower bed into a berm and he told me his mulch is what he uses in his outdoor pots, it would be a lot cheaper. I don't like bagged potting soil, it dries out to fast, so I mix mine half soil and half potting mixture. You can us packing foam in any shape if you break it up into smaller pieces.

We have a very large urn and my landscaper put an upside down plastic pot inside first to fill up some of the space and then filled it with a mix of perlite, vermiculite and potting medium.

Whatt we have used in the past for half whiskey barrels is empty plastic milk jugs or plastic water bottles. Makes it light weight to move and doesn't take as much potting soil.

Sue

In the pot at the house I bought was an upside down flower pot, to create an empty space, and less weight.

I would use a quality potting soil for planting. MG should be fine.

Moni

I agree about the styrofoam peanuts. But put them inside a trash bag. I then cover that with a circle of filter fabric and then put in about 8" of soil.

Yep packing peanuts work great or an upside pot...just about anything to fill the space. I like Miracle Grow soil, but for something really large, maybe you could mix it half and half with a cheaper one?

If you don't plan to move it, you could also use broken pieces of pots which you could get at a nursery.

I have used the black pots that the plants come in to fill up the big pots and packing peanuts. I liked the pots better, because when I dug out the old dirt, pulling the pots out is a whole lot easier than picking up the peanuts. (the bag had a hole)

littlebit

Real peanuts usually come in bags ...

... and I think that if you use the light-weight kind for the bottom of the urn, it'd be a good idea to contain them in a (probably garbage) bag, as well.

We have a hundred acres of dirt here ... more than likely the landlord'd let you have some of that ...

... if you want to come to pick it up ... but it'd be weighty, as you were concerned about.

We've been selling small quantities of rolls of sod (a square yard each) in recent days ... several visitors per day.

ole joyful

Thanks for all your responses! Unfortunately, I don't have any quantity of plastic jugs or packing peanuts left over. I will see if my local mailing store will sell them. That seems to be the way to go. I did have a question about the upside down plastic or flower pot used on the bottom. The urn still needs to drain, right? Does water still drain out around it? I assume, then, that it needs to sit somewhat loosely and not be jammed down to stop the water flow.

Empty soda/pop cans work well as a filler. As for soil in your containers, Walmart has a good potting mix, called Expert Garden, or something like that. It sells for about $7.50 to $8.00 a bag.

Make sure the peanuts you get aren't the kind that "melt" when wet. Also, depending on what you're growing, make sure you have good potting soil at least 10-12 inches deep.

It's best to put a landscape fabric, or old window screen, between the filler and the soil so it doesn't all wash out. You'll have to top it off next spring, until it all gets washed down and packed around the filler.

Make sure you post pictures once everything is growing!

I've used mulch as filler for extra large pots, and I've used the inverted flower pot method. Again, just make sure you leave plenty of soil depth for your plants.

If your upside down pot is clay, you don't need to worry about drainage. If it is plastic, cut out a couple of V's at the rim. That way, the water can drain.

Moni

Some pots have punch out circles on the bottom for drainage. When I retire from travel I am going to get two very large pots and put them under the coach lights by the garage door and I will fill them mostly with soil or the mulch I mentioned so hopefully it won't dry out so fast in the heat of summer.