Here, in PEI, a 12 to 18 inch tree will become larger than a 4 to 6 footer planted the same year by the end of year three. By year five, the smaller tree is still noticably larger. By year ten, the smaller tree may or may not still be larger, depends on soil and moisture. I should add that PEI has very good soil, so that the smaller tree will have lots of nutrients and water. This is backwards to what you asked, being as how, here, the smaller tree does better.

I understand your desire to get a better understanding of this situation, but I think you are looking at it from a way, way, way too narrow viewpoint. Just a few of the crucial parameters include species (which brings us another large list of factors), environment (including soil type, climate, and many other things), and relative retained rootmass. Trying to graph the phenomenon without taking the many other factors into consideration is like trying to graph the life expectancy of animals based only on how much sleep they get per night (it's mostly meaningless).

In general, and with "real-world" conditions, about the best you can say is that fast-growing trees will catch up with initially larger specimens much more quickly than slow growers. I've seen spindly one-gallon 'Green Giant' thujas, for instance, overtake large 6' B&B specimens in about 6? years. I would not expect that from a slower growing type of tree.

I think the key factor here, that may be easily overlooked, is the amount or root system retained at transplant. A small non-potbound pot-grown tree will retain approximately 100% of it's roots when it's planted out. If near optimal conditions are present during transplant, roughly zero transplant shock will occur and growth can continue without interruption. A larger B&B, or containerized specimen, or whatever may loose the vast majority of its roots before being planted out. In many cases, it takes a matter of years before the tree has recuperated from transplantation and growth is slow, or even very slow, during that time.

I planted more than a dozen 2-3 ft trees this past spring. The online nursery sold this tree in two sizes; 2-3ft and 5-6ft. Their prices for the specific tree I bought are:

$25.00 (2-3')
$59.00 (5-6')

So I bought the smaller sized trees, and by the end of October almost half of them were 6 1/2 feet tall. The others are 4 1/2 to 6 ft tall. It really depended on if they were 2 or 3 ft tall when planted. I had a mix of both. They are also not known for being fast growing trees; they're hardwood. I saved $510 by ordering the smaller trees.

Basically, by the beginning of fall I had the same size trees they were selling for more than twice the money. Plus, it's fun (for me anyway) to plant a small tree and watch it grow up.

As for what Brandon said, all my trees had HUGE root systems that weren't rootbound, so they retained everything they had when they were planted. I spread out all the roots as wide as they'd go, and we took the shovel and lightly loosened the soil on the sides of the hole to make it easy for the roots to grow through. And I watered the absolute heck out of them.

So that probably played a part in why they grew so well.

Some trees like oaks will get held back a few years to adjust to their new conditions. I have read that smaller trees can actually outgrow larger transplants because of this. In general oaks do a alot better if you just plant them as an acorn. Soil has a lot to do with it too if you got the right type slow growing tree will take off like a rocket and go beyond the known limits. If you hit all the right notes; organic matter,moisture,pH, drainage, and sunlight you will have a very healthy tree.

"Whereas its larger cousin may still be taking to its environment and naturally un-bounding and establishing its roots in year three."

From what I understand the tree does not 'un-bound' roots. It may grow new ones, but the old ones don't shed or straighten out.

Richard.

My opinion :
In general, 3-4' trees in 5 gallon plants are typically poised to grow the most and will within 3-4 years will typically outgrow larger trees.
18"-3' trees would be a close second.
The smaller trees "establish" quicker. Which means they have settled into surroundings and are growing on their own with little help. Compare this to larger trees where it will take 3-5 yrs before you can feel they have "established", if they don't die first.
Have a 4' Cedar Elm that in 5 yrs. is at 15'. An 8' Cedar Elm planted at the same time is still at 8'. Every yr. wondered if it would make it.
I grow just about everything from seed or small cuttings nowadays.
Plant small and enjoy their growth along the way.

Condition of individual specimens before planting is critical to the outcome. Old, starved stock that has been sitting around in played out potting soil may never bounce back, regardless of how good the final planting site is.

I like the acorn method if possible, no root loss and no waiting for the tree to get settled in etc. The 18+ inch oak whips I've planted take off year 2 in the ground. Those grown in rootmaker bags (air pruned) takes the guess work out of root prep before planting. Most oaks grow 2-3 ft in year 2 for me anyway in zone 6 with good soil moisture. In the south it may be more than 2-3 ft. I got many of mine at Mossy oak nurseries online. Prices are good. There are many varieties of oaks fom them. White oak Quercus Alba is going to take it's time getting settled in though, but it may be hard to transplant if it's too big. It (to me) is the north's answer to souths Live oak trees. It will after maybe 3 years getting settled,put on 2-3 ft growth per year. Chestnut oak Quercus prinus is very fast growing here even though it's in the wite oak family, most Red oak family oaks are fast growers, many live pretty long too,most of the white oak family oaks take a bit longer than the Red oaks initialy anyway, but they may live longer. Both red and white are going to outlive many popular trees though, you pretty much can't loose with a healthy oak transplant, Willow oak and Shingle oak can have blemished leaves in Summer, something I've noticed on them much more than other oaks (in photos) I've never started with a large tree to add that experience to this post.

If you can come up with this formula and have it be reasonably accurate, let me know. cuz I don't think it can be done.

among other things, location of planting can make a difference. a tree planted in MN or the central plains is going to have a different rate than one planted in a coastal location, and that's assuming weather that year doesn't mess with you. Weather will make a difference. plant is a year with mild night temps and plenty of rain but not flooding, and you are going to have a different story to tell than planting in...let's say Kansas in the year 2012, when the Mayan calendar kicked in.

I've planted 2" maples that I personally tagged and picked up a week later that suffered from transplant shock and I've planted 4" honeylucust and maples from the back end of the b&b lot (meaning they had been around for a while...like three years or more in a holding yard) that took off like weeds. I've planted bareroot oaks as large as 1 1/2" that grew like they were never dug and had problems with 4 footers.

The argument about planting 18" and having a plant of the same size by the end of the growing season just tells us how your plant grew. It says nothing about how a larger plant would grow. If you are satisfied with the results you got, great. If someone else wants to use different plants, larger stock...there is no right or wrong, just what you would do or not do, and there are many factors that go into that.

being a cheap bas..uhm, guy may only be one of them.

One of the things I love about the nursery and landscape industry as a whole is the incredible diversity there is to be had. Sure, sometimes my mind goes blank as I contemplate the vast differences that exist in the innumerable varieties of, let's say spirea or hosta or daylily, but also, you can get things that are incredibly cheap or incredibly expensive that fit the same piece that is the jigsaw puzzle of your landscape.

if you want a one gallon chamaecyparis cultivar, it's there for you. If you want a 36" version of the same thing, it's there for you. This is a diversity that deserves be celebrated. In exactly how many clothing stores do the high end fashions get stocked beside the low rent rags? I mean, other than goodwill (and I love goodwill). Do jaguar and ferrari generally share a showroom with volkswagon or ford? The marketplace diversity of the nursery industry is part of what makes it so dynamic...and that is to be cherished lest it disappear.

and there are different reasons to buy different plants of different sizes. just because I make one choice does not mean anyone else has to or that one is better or worse...it's just the one I made, and the one you made is just that.

I think not all tree are created equal, if we talk about fruit trees the 8 feet tree is more mature than the 3 feet tree. in terms of fruit production it will produce the next season while the 3 feet tree may need 3 season to produce any thing. As for shade trees I think I go for the cheaper ones.