Growth estimator

Why this is only an estimate

Two trees of the same species planted the same year can differ in height by a factor of two after twenty years. American holly, a slow grower, made 6 feet in 16 years under medium shade, while hollies mowed, mulched and fertilized for nine years averaged 22 feet at 10 years old. Soil depth, drainage, rainfall, growing-season length, competition, sunlight, seed source and whether the root flare was buried at planting all move the number. Read more in How old is that tree, really?, and on why published mature heights are often too tall in Why almost every tree height you read is too tall.

The model uses a typical early-life growth rate for the species' growth class, scales it for a poor, average and good site, and slows height growth a little at first and steeply near its mature height, stopping there. Diameter growth slows less than height growth, which is why old open-grown trees keep getting fatter long after they stop getting taller. Shrubs are projected by height only.

How the estimate is made

• Three growth classes. Each species takes the growth rate its plant page gives, and a species with none counts as medium. The classes follow the Missouri Department of Conservation's key: 12 inches or less of height a year is slow, 12 to 24 inches medium, and 24 inches or more fast. The estimator gives a young tree 1, 1.5 or 2.75 feet a year on an average site.
• Three kinds of site. A poor site grows at 0.65 of the average rate and a good one at 1.3 times it. Poor means compacted, shallow, dry or shaded ground. Good means deep, moist, fertile and open. The result shows all three, and your choice marks one column.
• A slowdown near the top. Each year's height growth is the rate for the site times one minus the square of the height's share of the mature height. So it slows a little at first and steeply near the top, and it stops there. The mature height is the top of the height range on the species' plant page. With no species it is 12 feet for a shrub, 30 for a small tree, 50 for a medium tree and 85 for a large one. A tree entered taller than that keeps its height.
• The trunk. The trunk adds 0.25, 0.4 or 0.6 inches of diameter a year by class on an average site, scaled for the site like the height. It slows less than the height, to 0.6 of its rate at the top, so an old tree keeps getting thicker after it stops getting taller. With no trunk entered, the estimator starts from the trunk its own rules give a tree of the height entered. Shrubs are projected by height only.
• The first years after planting. A planted tree spends its first year or two growing roots while the top waits, and the estimator gives it the full rate from the first year. Enter the height your tree had in its second or third spring, or expect a new planting to reach each height a year or two later than shown.
• Your site from a twig. On a tree past its first two years, last year's growth runs from the tip of the main stem to the first ring of bud scale scars. A slow grower makes about 8 inches a year on a poor site, 12 on an average one and 16 on a good one. A medium grower makes 12, 18 and 23 inches, and a fast one 21, 33 and 43. The column your tree matches is the site to give the estimator.
• Size when grown. The width when grown and the distance from a house are the plant page's own figures, the same ones the planting-site check uses. They do not change with the years ahead.
• A height to watch for. The year a tree passes a height comes from the same yearly figures, for each kind of site. A height above the mature height is never passed.

The growth classes follow the Missouri Department of Conservation's growth key on its Tree Growth Information page. A slow tree has "an annual growth increment of 12 inches or less", a medium one "between 12 and 24 inches of growth per year". A fast tree has "the potential to grow 24 inches or more annually". The rate inside each class, the site factors, the slowdown, the trunk rates and the trunk read from a height are the estimator's own reading, nursery rules of thumb rather than measurements. The department rates hawthorns slow and black walnut medium. Here the plant pages decide, and they put Washington hawthorn in the medium class and black walnut in the fast one. Each species' mature height, width and distance from a house come from its plant page. How fast native trees grow gives measured growth for the species we grow, the first years after planting and the twig test. It also shows what soil, sun, weeds and deer do to a young tree. It names its sources, among them the Forest Service's Silvics of North America, whose American holly chapter gives the holly figures above.