Don't know if you can do this with the trees in the pots. Normally you would turn the pots up side down, wet the pots so you have a damp surface and add an acrylic fortifier to your mix. After the bottom and sides have setup you turn the pot over and finish the return on the inside of the pot. You only have to cover down to where the soil would start inside the pot. Since your pot is starting to deteriorate I don't now if it can stand up to the removing of the trees or any of the other process.

The only pot that has ever cracked was a concrete one that I did not make. It was under the deck on the back of the house and full of soil which froze and broke the planter. This planter had straight sides so it wasn't a matter of the sides slanting in. The directions I got with the planter from the manufacturer said not to leave it out over winter with just soil in it and if planted and sitting on the ground to break contact with the ground with a piece of wood. I have a couple with trees and I shove a piece of wood under them. Most of my hypertufa planters have trees or shrubs planted in them and they stay out all Winter. I use equal parts peat moss, sand and portland cement for most planters. By the way, even though that concrete planter cracked I really liked it so I still use it. I plant vines to cover the cracks. I have some nice size concrete urns from the same manufacturer. Their in Quakertown, Pa and they use to have a bargain area for concrete items with defects. I haven't been there in a while and I understand this place is only open on the weekends now.

I am also one of those people with 500 plastic pots. I have square pots that fit snugly into a flat so I guess they must be about 6 inches square, because I can fit 8 in a flat. They are deep so there is plenty of room for soil. I tried putting plastic covers down over them and eventhough they had finished corners they still allowed too much air flow and were drying. Part of this was because they were on a shelving unit and not on the ground, but partly because they didn't fit snugly enough. I solved the problem by buying clear plastic bags that are made by Glad and are called tall kitchen bags. While the bags were still folded, I took a hand held hole punch and punched many holes in them. They are big enough to slide the entire flat inside and still tie loosely at the end. I needed something that would hold the plastic off of the plants. Seedmama suggested metal mini-blinds attached in the center with a paper brad and placed in an X shape over the flat. I think this is an excellent idea but had gotten rid of all of my blinds before I realized their usefulness. The few I had, I used for tomato markers last year. Since I haven't found time to run to the thrift shop to find a blind, I took the rings from my old broken tomato cages and stood them up in the center of the flat between the pots. Those cages are not big enough for tomatoes anyway but I use them for peppers. This ring created a mini hoop house effect but of course the cover has holes in it all around. I wish I could put mine on the ground but there are just too many loose animals where I live. I have already had to run a cat out of them before I got the hoops in. He saw a basket and decided it was a cat basket, I guess. It was before any seeds had germinated so no damage was done. Damage might have been done (to the cat) if I could have caught hime at the time. LOL So....if your pots are on the ground you might be able to cover them tightly enough to work OK, like the pic above in the bread trays. If they are not on the ground, I think you will have to put them in a bag with holes. Just my 2 cents. If this is not clear, I can take a picture. Carol

I made hypertufa rings to set in mygarden. The edges of each ring stands about 8 inches above soil level. No bottoms, just the rings. My husband cut out the molds from a heavy black plastic tapered nursery plant pot by rolling it along the blade on his table saw. One ring from the top of the pot and the second ring from the bottom of the pot so there was about a 2-3" space between the two rings where I packed in the hypertufa mix. I laid the smaller ring inside the larger one on a piece of plywood covered with heavy plastic so nothing stuck. When cured for about 6 months outside in the weather, I set rings into soil and planted them with hostas and sedums. All thrived, I think because they could grow into well draining soil rather than into a container with a bottom. Even with drain holes, I think they still accumulate too much moisture in the winter for some hostas. One could make these planters square or round. Carve a design in rim and outside when dry to green stage, about two days after molding them.