Ok, I’ll complicate things even further. Let me say up front, this is just the opposite advise I provided to folks planting bare root trees in heavy clay. It is a method I use for planting trees that I grow from seed in root pruning containers (Rootmaker in my case). This does not apply to bare root trees or other containerized trees.
I won’t go into root pruning containers here, there are other threads were folks talk about the plusses and minuses. They work very well for my application and area. Unlike bare root trees which pretty much be planted when dormant, these trees can be planted in the dead of summer if supplemental water is available and don’t have the years of sleep, creep, and then finally leap. They can continue growing immediately when planted because the root ball is completely undisturbed when planted.
I typically plant from 3 gal RBII containers but the same technique works planting form 1 gal RB2s. I’m planting hundreds of trees each year on my farm for wildlife. These trees don’t get any supplemental care. I live in an area that has pretty reliable rain in the spring and fall/winter but can have hot dry summers.
First, I select a site where ground water will not drain into the hole. I then use a tractor with a 3-point hitch auger to dig holes. The diameter of the auger is just slightly larger than the container. I dig the holes deep, between two and 3 feet. Depending on soil conditions, I may use a hand rake on the sides of the hole to mitigate any glazing. I next backfill the hole with amendment. I actually use quarry stone because of the low cost and high volume I do, but I’m sure there are many much better amendments that folks can use. The key is that it is fast drainage.
The RB2 containers unwrap so the dense root ball and medium are completely intact. (I use a 50/50 mix of Promix and mini pine bark nuggets which work well with these containers for growing trees). I backfill the hole (mixing the quarry stone with some of the native clay) until the root ball sitting on the backfill is about 1" - 2" above the soil line. I then take the native clay and mound it up over the medium creating a slight mound. I apply air and water porous landscaping material and use quarry stone on top of it as mulch.
I found no one else with this exact high volume clay soil application, so I’ve tried to come up with an efficient low cost method that applies the principles in the papers I’ve read on infiltration. Here is the theory:
The combination of the clay mounding and landscape material help prevent excess water from entering the hole. The depth of the hole and amendments below the root ball allow any water that enters the hole to collect below the root ball and not drown the tree. Whether I plant in the spring or the fall, immediately after planting I have ample rain. This time before a dry period, along with the narrowness of the hole, gives the lateral roots plenty of time to grow into the native clay which hold water well. So, when our summer comes around the trees do fine.
I can’t vouch for the long-term effects of this, but I’ve been planting trees using this method for about 4 years. These trees have done better than other methods I’ve tried. I’ll need quite a few years more to make sure there are no negative long-term effects of this method. So far, so good. Again, this is only for trees grown in root pruning containers.