You MUST inspect the root system if they have been in 5 gallon containers and are that tall! I fear you will likely end up returning them once you find out how messed up the root system is. You will probably end up removing inches of potting soil before you even get to the root flare (the flare is your aiming point to get level or just slightly higher than the surrounding soil). Largish oak roots (relatively speaking) are pretty stiff in my limited experience and I have had trouble trying to straighten them out at planting. Surely for $165, they are in a bigger pot than 5 gallon? Any warranty with them? I would go ahead with planting if they are warrantied. If no warranty in that small a pot, I would return them without messing with anything.

Any way you could post some pics for us to see the trees and their containers?

John

Just to give you a comparison, I raised several "Burs" from acorns in 5 gal. pots. After sprouting in the spring, most were about 1 - 1.5 ft. tall when planted out the following winter. Several had sunk roots out of the pot and into the ground and had to be cut off.
For yours to be 10' tall and still in 5 gallon pots is somewhat interesting, but indicates that the tree roots are grossly undersized for the amount of top you have.

I am not entirely sure if it was 5 gallons or a little more. The trees looked really healthy. The flare was above soil level. But now that you guys are telling me I am really concerned that I might have just made the worst possible investment.
How can I make sure that the plant get a long life after having grown in such conditions?
The plants have not been delivered yet. So maybe I can cancel my order.

I (and many here) usually recommend small trees with small root systems if they are to be mail or internet ordered. This way the root system is smallish and workable when the tree needs to be planted. If larger trees are bought and delivered in pots too small (happens very often, even at nurseries), negotiating the rootball can be very challenging and sometimes impossible.

I would make sure I could examine the roots before being stuck with the trees. If the delivering party was not OK with that, I would cancel my order pronto.

John

I saw yesterday a 5gal pot and it was definitely smaller than the one I have seen the burs in. Probably the trees were in 10-15gal pots

Should I loosen all the soil and expose the root system before planting them?

Cheers

That's a little more comforting. And yes, you still need to loosen the outer soil and examine the root system. And don't forget to take some pics for us if you get a chance! It can be hectic and stressful when dealing with potbound rootballs so I totally understand if you can't take pictures while messing with them :)

John

Do not loosen those roots. Messing with the soil around the roots is a common mistake people make when planting trees and shrubs. The only time you should mess with the roots is if they are encircling roots. If you see encircling roots growing all the way around the outside of the root ball, you will want to carefully cut them away from the root ball.

http://www.gardenguides.com/138483-plant-container-grown-tree.html

A lot of people actually wash the potting soil off of the rootball and plant container trees as a bareroot tree. This can be done with the best success when the tree is dormant but if attentive aftercare will follow, it can be done after leaf out but will result in stunted growth for a period of time, possibly the remainder of the growing season. You CAN NOT SEE through the soil and therefore must remove some to make sure the root system is adequately healthy and not tangled into a ball just beneath the outer potting mix layer. All that will happen is you native soil that is the backfill will occupy the space previously held by the potting mix, actually benefiting quicker establishment of the plant.

John