I'm going to plant two pecan trees. When I plant them, they'll just be pretty small, but will have an ultimate width of between 75 and 100 feet years from now.

Due to flooding issues, I would like to raise them up 8-12", and to prevent dirt from being washed away, I intend to essentially make an wooden raised bed, 2 1/2 ft square, 8-12" tall, and plant the tree on the top of that.

I'm hoping the root system as it grows will grow down and then spread out, but I wanted to double check that the roots won't be damaged by the constraints of the wooden raised bed.

Worse comes to worse, I can just unscrew and remove the raised bed after three years or so, but my question has to do with me leaving the raised bed boards permently in place.

Can I plant a tree in a open-bottomed raised bed, even if it's from a larger tree species?

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    A growing tree is more likely to destroy your raised bed than suffer any harm itself. A tree's roots tend to spread our horizontally just under the surface a little further than the extent of its branches. Either the roots will be constricted by the raised bed, which would affect the tree's stability, or more likely they'll find a way out, either over, under or through the sides of the raised bed. I guess you'd prefer them to go under, in which case the raised bed would be a bit moot. What's the nature of the flood risk? How is the surrounding area landscaped (is it paved, asphalt, etc?) – David Liam Clayton May 12 '17 at 8:47
  • It seems to flood annually, for a few days. I lost a couple trees in previous years, but others that were on ground just a few inches higher have done well. The surrounding area is grass and forest. It's planted between a mini forest and a grassy field. I'd like to keep planting in this location, if I can keep the trees alive long enough to get established. – Jamin Grey May 12 '17 at 8:56
  • Surely the ground where the tree's roots are will be saturated, whether or not the tree is planted on a raised bed? (assuming the tree's roots won't stay in the raised bed but spread out beneath it). If you think that soil washing away will be an issue then you could cover it with landscaping fabric or plant some ground cover plants whose roots will bind the earth (might be hard directly under a tree). I'd also try to ascertain the flooding tolerance of pecan trees - different trees will have differing abilities to cope with sitting in water (presumably we're talking about fresh water). – David Liam Clayton May 12 '17 at 9:52
  • It seems to me, as an gardening newb, that if there is standing water on the trunk - even just an inch up - or if the ground around the root ball is hyper saturated, it greatly reduces the chance of survival. If the trunk and rootball are lifted above the flooding, I think it'd greatly improve survival, and also reduce the number of days that the ground is soaked - seeing that the higher ground will dry first. Roots will ofcourse descend down into the flooded level, but that'll take a year or two of growth first, and the bigger the root system, doesn't it become more resistant to drowning? – Jamin Grey May 12 '17 at 14:23

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