How big are the mango trees? If you have to move moderately sized trees in and out every year (as you must in Canada?) then I would suggest you replace the granite with something else. The only reason I would use the granite for a tree in a pot larger than ~8" is if you're worried about it being blown over or etc. It just makes the pots far too heavy to deal with. I ended up using ~1-1.5 bags of the gran-i-grit which put some pots at 75 lbs just for the granite component.

The granite can be replaced with similarly sized perlite, charcoal, or anything else that doesn't hold significant quantities of water. Perlite would be the lightest option but you should check to see if mangos have any known issues with fluoride sensitivity (it's only an issue for a few types of plants). If you use charcoal make sure to wash it well to begin with and handle it gently as it is obviously more fragile than the granite etc..

I've used reptibark in the past and it will work - it is probably slightly larger than ideal and it is *very* expensive for what you get. You might even be able to mail order pine bark for less than you'll pay for the 24qt reptibark bags necessary for a large tree...

I didn't buy the trees yet, I will buy them in spring time ,but they should be in 7 gallon pot.I just find out the repti bark and it cost me 12.99$ for a 8 quarts bag.Should I get a refund?They don't sell any 24 quarts in my pet store.For me the weight of the pot is not really a problem.I know it's not ideal but I can deal with it.I went to home sepot and I found some rocks ,but it looks like a mix of crushed granite with silica sand.It's a bit larger than I was looking for so I'll sift the larger parts.

Where could I order pine bark?At this time of the year all my garden stores are closed.

here's some pictures of the rock mix:

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