The optimal plant size

After having planted hundreds of trees in the Putt Myra woodland garden, I have observed that small trees generally have better than larger trees that I bought especially during the first years of the establishment. The small trees survived drought, survived vole infestation when the big trees tvärdog and many have now grown on trees that were pretty big from the beginning.

To find out if this is a coincidence or a general phenomenon I have tried to find answers in the scientific literature. Unfortunately, most of the research in tree establishment made the really big trees that are used in, for example, parks and urban. I do not know of a single forest garden projects where trees of this size and price range has been planted, which probably has to do with the tree with for example a diameter of 5 cm at breast height can easily cost two- to three thousand kronor. Nevertheless, there are some interesting finding in the literature.

One of the most renowned scientists in the field, Todd Watson from Texas A&M University, believes that the general conclusion is that small plane sizes are preferable from an establishment viewpoint [1]. The reason is that the large trees have a huge imbalance between the root system and crown. When the trees are picked up before planting out, they lose up to 85% of its root mass and it takes years for them to restore balance. In our northern climates grow roots in the average 30-60 cm per year, according to a rule of thumb presented in the research report takes for each 2,5 cm in trunk diameter a year for shots- and root growth will be at the same level as before repotting. For a tree with a trunk diameter 5 cm, it takes even two years so to say, to catch up with itself and begin to grow normally. And it does so only if both get enough water and nutrients, which means greater care needs during the establishment years. A small tree with a trunk diameter 1 cm, in theory, therefore, only slightly less than five months to begin growing as if it had never been transplanted, but unfortunately I have not been able to find any research on trees with these dimensions, then they are rarely used in landscape plantings.

Even for plants that have been drawn up in pots, it seems valid that it is beneficial to small sizes. In a fairly recent study from the US found that "trees from pots in small sizes are affected to a lesser extent by planting stress, they have a shorter etableringstid and increased growth rate " [2].

What complicates the matter is that small trees are said to be less hardy than larger trees. For us, growing on the edge of what is possible is an important aspect to keep in mind, Perhaps more important than the rapid establishment, In any case, when it comes to sensitive trees. So where does the lower limit, where we get a good compromise between plant size and increased resistance? I asked this question to Todd Watson in the US but unfortunately got no clear answer, more than "the smaller the better". The only hands-on report, I have been able to find how much thermal resistance of trees growing concretely with age is about young stands of rhododendron [3]. According to this study increases the thermal resistance dramatically from seed germination to two to three years and continues to increase to the trees is five years old. When the trees have passed 10 year increase frost resistance significantly slower, but still, trees become more and more resistant with age.

Should we assume that the same applies to other more interesting species as walnut (Juglans spp.) chestnut (Castanea spp.) eller pawpaw (Asimina triloba) I for my part have concluded that it is best to have these rather sensitive plants in pots for two to three years before I plant them in the open. Then winter I use them in my cellar and schools for them a couple of times so they do not get rotsnurr or a poor balance between the crown and root system.

I look forward to hearing what you readers draw the appropriate conclusions and if you have other sources related to the topic do I gratefully accept tips on these!

Sources

[1] Watson, W.T., Influence of Tree Size on Transplant Establishment and Growth. HORTTECHNOLOGY-Alexandria vari-, 15: p. 118-121, 2005.
[2] Garcia, LM, M.A. Arnold, A. Texas, and M. University. Post-transplant establishment and economic value of three tree species from five container sizes. 2015. http://hdl.handle.net/1969.1/155080
[3] Lim, C.-C., S.L. Krebs, and R. Arora, Cold hardiness increases with age in juvenile Rhododendron populations. Front. Plant Sci. Frontiers in Plant Science, 5, 2014.