I've also had this happen but I've never had a problem with freezing temps. It only happens with the stuff I've made in the desert. I have a winter home in Yuma, Arizona and there's very little humidity in the air. But, it usually only happens with the bigger stuff. My small stuff rarely cracks.

Is it pots that you recently made and what recipe are you using? Are you doing anything different when you make them? Different ingredients?

Yes, they're pots and they were probably 6 months to a year old when I planted them. Two of my big pots are cracked and one of my smaller square pots. I usually use 3 peat to 3 perlite to 2 Portland cement. I don't think the recipe was any different when I made these, but you know, they're always a little different.

Could you give us all the details? It helps to try to diagnose the porblem if you tell us more details? Did it freeze in your area last Winter? What USDA zone are you? Is there soil in the pots in the Winter? Anything growing in the pots in the Winter? How thick are the walls? You use no sand or reinforcement? Is the peat moss clumpy or sifted? If it freezes just one night and all the planets collide (soil in pots, nothing planted, pots in contact with the ground) you can get cracks.
These aren't your two part urn pots that you made a while back that you joined together are they? They were a different recipe.
Any kids or animals or significant other that knock them over? LOL!
As you can see there are lots of things that can cause cracks and I didn't cover all the reasons.

How long do you allow them to cure?
Mike

Hi - Sorry I haven't gotten back sooner. So much to do this time of year...Let's see,in answer to Billie's questions: Yes, it froze here last winter. But I live in the Pacific NW and we have short freezes (i.e. not cold for extended periods of time), but nothing like the Midwest or East. I live 7 miles from the Ocean, so I think I'm in Zone 8 (to 9). Yes, the pots that are cracked had plants in them over the winter - but I did have two pots that were just curing over the winter and I went to plant them in the spring and they broke....I have added sand, but I'm not sure if the cracked pots have any sand...I don't sift the peat; I do sift the Portland cement.I usually add white glue to the water when I mix. My pots are usually about 1-1/2 to 2" in thickness and I cure anywhere from a month to 6 months or more.

What I really don't get is how come some crack and some (MOST) are just fine???? If I can figure out how to post pics on here, I will!

Thanks everyone! I'm hoping things settle down so I can get back to my hypertufa and leaf casting! Maybe next weekend. :-D

Could it be a pot design issue?

The least likely design to have freezing problems is one that is smaller at the bottom than the top, with outward-sloping sides. When the contents freezes, it tends to expand upward and outward, following the design of the pot.

The most likely one to break is the one that is built like a cauldron, with a smaller bottom widening outward in the middle, then shrinking down again toward the top. The freezing contents become trapped and expand beyond what the pot shape can tolerate.

The other problem is tufa itself. Organic materials rot away (peat, coir, sawdust, etc), leaving channels that water can move through and into. Some of it gets trapped when a freeze hits, and all those little bits of swelling ice weaken the tufa overall. A little bump the next year, and here you've got several pieces of one pot.

If you've got a pot you're really proud of, plant into a removable cache pot, and put the tufa pot in the garage or shed for winter, after drying it out as well as possible.

You can fight the laws of physics all you want, but you will lose. Just plain water carves canyons, rain wears down mountains, and tiny little bits of ice breaks pots. It's the Law.

Sue

I am new to this forum, I am looking for classes on hypertufa planters and cement leaves classes.
Anyone having classes in south florida????
Let me know !! Thanks Linda

"I cure anywhere from a month to 6 months or more."
Wow - How to you keep the container damp for 6 months?
It is not necessary to allow them to cure - ( remain damp in a plastic bag) that long, 2-3 weeks is just fine. Also when you make the pots - the temps should not go below 50o F during the mixing or curing process.

Linda - welcome. You may get a better response to you question if you post you own message rahter than reply to this post. Look at you local garden centers, or local garden clubs - you will find classes. However there is so much info here - you can learn from the readers.

Mike

Is there a way to repair my hypertufa bowl after I tried to unmold too soon? I tried to patch it with more hypertufa...don't know if that is going to work, I don't feel good about it. Should I soak or spray with H2O?