I have tried growing cacti and succulents in Minnesota and some have made it while others didn't. 1. Pediocactus simpsonii( Mountain Cactus) had 3 plants and lost two during one summer, then lost the other one the following summer. I know it wasn't the cold that killed them since they grow high in the mountains and most of the growth is during the spring when it is still cold. I don't think they received enough water in the summer and/ or it could of been the heat. They are known to despise heat especially humid heat. They didn't look dead after the winter so I doubt it was the cold. Im guessing they require more rain than other cacti and in the mountains there is a sufficient amount of rain for good growth. 2. Opuntia imbricata- Survived one winter here but that was it. It could be because all my cacti arn't planted in a raised/elevated bed. The soil is amended though and there is a 1 foot layer of just rock at the bottom of where they are planted. 3. Opuntia polyacantha " Alberta, Canada" - These have survived 2 or 3 winters now and im suprized since Alberta, Canada is a much drier climate than Twin Cities, Minnesota. 4. Opuntia compressa(O. humifusa) These have also survived although one plant it either rotted or a snowstorm or rainstorm damaged one of them during the winter. I took a pad which broke off and was laying on the ground in early march and replanted it indoors in a pot. I put the plant outdoors in May. Now I have two plants rather than only one (I originally started with 2 before the damage). They are said to like cold and wet but O.polycantha has done alot better. 5.Echinocereus viridiflorus - collected from Hot Springs South Dakota. Has survived 2 or 3 winters now and is very small still. 6. Agave utahensis- Peach springs, AZ. It didn't even make it 1 winter. Im not sure if it was the cold or the fact that it got wet when it was cold. It may of survived if planted on a slope or raised bed in rocks. All of my plants are on the south side of the house next to the house so it is warmer and they get alot of sunlight during the summer and winter. They arn't planted in the middle of the yard! So it is kind of a microclimate. I started all of these from seed and didn't plant them outdoors the first winter. I am thinking of trying Yucca glauca , Agave parryi, and Escobaria vivipara. I will have to make an elevated planting area or cover the Agave in the winter. I think Agaves dont mind cold it is the combination of wet and cold that usually kills them. New mexico can get very cold but at the same time it is dry. Florida can get very wet but usually it is warm. Agave americana can grow in Florida and Cuba despite alot of rain but you have to remember it is usually not very cold in those places. Agave americana can also grow in the Southwest where it gets very cold but it stays dry through the winter there.

One would want to find out which way the floor/ceiling joists under the floor ran and made sure the wood beams Spanned some of the joists to either side of the container to insure the weight would be distributed to other weight bearing structures.I do not think that the added weight of the wood would be much and the added benefit of the weight spread would be good. I would consult an engineer and find out the weight in these containers. I don't know what your floor plan of the the area usage is and if this would work in your situation. I also do not know how much money is available for your project. I would think an engineering STUDENT could answer these questions and would love a little cash to be pressed in their hands. I got an architect to help with my house by a note on a bulletin board at the university. If one did not like the look of the Void underneath the raised containers, An artisan could grind /cut slots that the beams could slice into keeping the heiht the same as placed on the floor. This would have to be done by a GOOD metal worker with proper tools or one would have a mess of raw jagged ill fitting metal. Then one could use the top of the beams to place the rubber fountain /swimming pool structure. OR, One could add and rivet on a metal fill pieces on the bottom edge to carry the metal cylinder to the floor but this would make the metal cylinders look taller and increase their appearance of vertical mass. Anyway you do this, I would have it drawn out so you know what one is getting into. God, I need to shut my designer mind off.It has a tendency to raise away. I will probably talk you out of it by discussing difficulties and possibilities. It could be a very fun project but it could also be a hair puller if approached wrong. Styrofoam will not break down but some very knowledgeable professionals on another site said that Styrofoam does leach chemicals that plants don't find beneficial. I forget what chemicals they said or the effects on plants. I am just repeating a remembered conversation.If they are placed on the bottom and one has provided a fast draining soil, that leaching should be going out the container and not ip wards. Roots do find their way into every corner though.

"When choosing plants for the bed, choose those that grow in full sunshine. Since plants grow most actively in the spring and summer months, anything you plant there will need to be tolerant of summer sunshine. The winter shade should not affect sun loving plants at all." This is true if you have spring summer growing plants that can tolerate the USDA hardness zone you forgot to mention. Example the east side in zone 5B adobe here gets very little sun during winter the same east location receives enough sun to keep many " full sun " plants happy from Late April early May to early mid October. To include Apple trees cherry trees hydrangea juniper and even roses. ( All PIG's of course) In theory a dormant plant doesn't need much of anything and often does include " full sun". Chances are; even with a solid transition the sunny summer east side here would easily fry the Euphorbia, Agave, Aeonium you mentioned before mid June if they where in a pot. If pigged they wouldn't make it though the winter for reasons of the USDA hardiness zone 5-B IMO as you mentioned " BED " at least make some mention of your USDA plant hardiness zone.

Thank you all for sharing your wonderful photos. I loved seeing every one of them! I don't know what Mother Nature has in store for us this summer, but this is truly a wonderful spring. It's amazing how the land can recover. This photo was taken right after things were cleaned up from the "big freeze". Whew, that was a LOT of work. The freeze and the hail storm was a double whammy for some of us. Here things are brightening up. In the left foreground notice the mound of white wine cups that have taken over the planter. A couple of flowers have opened and the mound will soon explode in bloom. Thank you Mara! Both of my furry garden helpers are in the photo. Can you spot them? Columbines ... Wooly bee bush doesn't waste anytime in starting its almost year round bloom ... The old man of the tree seems a bit surprised that I planted begonias in the little fountain. I'm amazed that the night blooming jasmine in the pot on the left survived the freeze with no protection. It probably helped that the roots go into the ground ... To 'replace' the irreplaceable :-( shoe string acacia a little rusty black haw viburnum was planted. Thankfully the grey dyckia from Eric survived the freeze. It was unprotected. Mexican feather grass and four nerve daisy require no protection, nor did the Satsuma citrus against the brick wall. Agave blue glow in the big pot was damaged, but per Mara's suggestion I removed it and let it dry out before replanting ... Well, the clock changed while we slept, but contrary to appearances it's still not 'officially' spring. Could've fooled me ... :-)