INDETERMINATE GROWTH & Paniculate Hydrangea

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A common gardening misperception is that there is a genetically predetermined full-grown size for woody plant species (i.e., stop getting bigger while still living). What is generally called mature size. Fact is, ALL woody plants (trees, shrubs and vines) have indeterminate growth whereas humans, dogs, cats, etc. have determinate growth. Moreover, how long a plant can live is variable -- a combination of its genetic constitution (i.e., species specific) and affected by environmental conditions. The idea of a prescribed and appropriate/preferred size is what most people want to hear -- the plants as furniture crowd -- but that is not true. If you want an idea of potential size for a woody species multiply expected life X amount of new growth per year but realize there can be variation in both these factors. Thus, for example, an Eastern red cedar located in perfect conditions will grow faster and get bigger, but may not live as long as one growing on a rock outcrop. Species and circumstances will determine longevity. But how to tell for a recently created hybrid/cultivar whose parent's longevity vary and with both having the potential to live long (e.g., Green Giant arborvitae)? The growers/label makers guess, favoring a number that will not scare the customer, usually implying a ten-year average, about which the public is clueless, and then deceiving their customers by adding MATURE SIZE.

Woody plants have two types of growth. Primary growth is an increase in height (length) and width/canopy (again, length) due to cellular division in the apical meristem -- a tiny dome of mitotically active cells at the tip of stems that produces new cells pushed off either upward or downward (mostly), forcing the tip away, thus the increase in length. There is a corresponding zone in roots but its new cells are mostly pushed upward thus forcing the roots to elongate down and outward. Secondary growth refers to an increase in stem/trunk diameter mostly due to cellular division of the vascular cambium/lateral meristem -- a thin encircling band of mitotically active cells located just under the bark. The vascular cambium produces new cells which are pushed off to the outside (then differentiating into phloem, sugar and other nutrients conducting cells) or to the inside (xylem, water conducting cells). I said mostly because there is yet another mitotically active layer/band outside the vascular cambium and phloem that produces the epidermis in stems and roots. This additional layer, called the cork cambium, has an ever decreasing impact on the measure ratio, in part because that tissue can be sloughed. Furthermore, the stem tip is where Indole Acetic Acid (IAA) is produced. IAA is a plant hormone, an auxin that among other things suppresses lateral growth. By removing the tip(s) the source of the auxin is removed and the suppression is released -- the reason we pinch stem tips to induce more lateral (bushier) growth. One would think this fundamental knowledge important enough that it should be required learning in school and by all gardeners. Alas, these facts may not be "sciency" enough to merit focus in our new biology. I feel fortunate to have lived in a time when a class like Plant Anatomy was still offered at university even though I discovered about indeterminate growth long after my formal botanical education.

Here is another good reason the general population should know about indeterminate growth. Every community has to deal with utility line problems caused by vegetation, especially trees. Many of these trees were planted by property owners -- directly under or too close to the lines, not realizing the tree(s) would not stop growing and thus become a trouble. There are many hundreds of such examples just within a half mile radius of my residence, including lots of recent installs (e.g., a staggered row of 12 Green Giant arborvitae; see my Sept 2021 rant). People do not know, want to believe or abide by the botanical facts or the regulations regarding utility easement (right-of-way) corridor activity (see Chapter 22 in my Ranting book). As a result, dangerous and frustrating power outages occur and the utility companies pass the cost of that repair on to us, the customers. For Indianapolis alone, the annual line vegetation management expense for the main utility was $10,000,000 a decade ago, and that does NOT include the additional expense associated with non-scheduled maintenance (i.e., emergencies). FACT: while the rules can vary from community to community, utilities generally have the right to access and remove any vegetation they deem a potential hazard in and above the corridor, and they can legally do so without notifying the adjacent property owners, although they often do.

I will now use a common woody ornamental species to illustrate facts and falsehoods about woody plant size. The above pic was shared by my buddy Chris Wilhoite (Soules Garden, Indianapolis) who saw it along with the accompanying text on Facebook. I do not use social media, nor do I know the people shown, but I can tell you that the pic was taken in extreme northeastern Tennessee (Shady Valley) over half a mile high in the Appalachians. The region has/had places that easily qualify for the descriptor Paradise, many of them now ruined by humans. We have that spoiling behavior in common with swine. Further, I can inform you that the featured plant is a perhaps century-old and nicely situated paniculate hydrangea (Hydrangea paniculata) and is likely the cultivar 'Grandiflora' AKA peegee. The names PeeGee and PG are two other shortened forms of the combined species and cultivar names. The species is native to China, Japan and eastern Russia and was introduced to the U.S. in the 1860s. The origin will likely turnoff some native plant zealots even though the species does not escape cultivation (see several past blogs, esp. Jul & Aug 2023, May & Aug 2022 and Jun 2019). SIDE NOTE: The musical group the BGs, later the Bee Gees, was named after Barry Gibb, the oldest brother of the act and for Bill Goode, their early promoter. Coincidentally, but not actually, the name also matches the brothers Gibb. Moreover, I am confident the name was also based on the preexisting similar moniker of this popular hydrangea.

The magnificent specimen shown above, which has not been managed by pruning for many years, except for the lower portion, is 30 feet tall, has a spread of 35 feet and a trunk nearly two feet in diameter and, barring pruning or death, all will continue to increase. This is the same species too many people tuck up against their house foundation, often under a window, because they read and believe the "mature size" BS. As I have said a thousand times plus, and repeatedly note in my Rantings book and these blogs, no woody plant (tree, shrub or vine) stops growing unless it is dormant or dead. The rate may change but growth will continue if alive. The only exception/variation is partial and only applies to a small group of shrubs (e.g., sweetspire) and even fewer trees (e.g., sassafras) that divert some of the new growth (clonal) to lateral expansion of the clump via new stems. But that is not what people are told or want to hear, so they ignore reality and misplace/misuse the plant. The plant's genetic constitution and growing conditions determine the potential/natural size, not a person's fanciful want. -- I seem to spend half my time telling people real things (reality) they don't want to believe, even though they would benefit from listening and abiding. Part of the reason I assumed the designation Mad Botanist.

To validate my claim about misinformation (the BS) regarding size I typed peegee hydrangea size in the internet search bar and got the following "mature height" info: (1) 12"-18", (2) four to ten feet, (3) 8 - 12 feet, (4) 25 feet, (5) 10 to 15 ft., (6) 10 feet, (7) 10-25', (8) 10 to 20 feet and (9) 4 feet. See what I mean? Yet most people accept this "info" without question -- "a man hears what he wants to hear and disregards the rest" (Paul Simon, The Boxer). I have heard it referred to as "happy ears." Regarding the size, either the various sources (1) do not know or (2) they are lying to us and need to get their story straight. Either way, we should NOT consider any such information source reliable. You should also note the location of the source. Why would you shallow whole "advice" concerning plant cultivation given from/for a maritime climate with sandy soil if you live mid-continent on clay? Foolish, but common. By the way, foolish was the word of the month for Jan on the roadside sign I display at my place.

Mind you, it is possible to maintain a much smaller peegee, or any of the several other cultivars of paniculate hydrangeas, which includes some recently released dwarfed types, but it requires an annual hard prune. I do the pruning in early spring but it can be done any time after the aerial portion of the plant goes dormant in early winter. Since the gorgeous inflorescences of this hydrangea are produced on new wood, early spring pruning will not prevent flowering. In addition to keeping the plants smaller, regular pruning will promote the production of fewer but much larger panicles. HOW & WHEN: I remove last year's growth to just above one of the lower nodes (location/from which one or more leaves has or will emerge). I also make the cut close to (i.e., just above) a node to prevent creating unsightly stubs. I also remove any branches with a diameter less than pencil thickness. The effect of the pruning on the whole plant is the production of a slightly domed platform from which the new growth will extend -- expect about three feet. I typically do the pruning mid March - mid April. As noted, it can be done earlier but I find the dried tan-brown spent panicles charming in their own way. Older leggy specimens can be recovered by hard pruning, but if the specimen has been unmanaged for too long it may take a couple of years to get it under control, i.e., shaped, sized and flowering as you want.

The standard (lollipop) form is popular -- and standard is the actual term -- but I find them unappealing, especially since I rarely see them pruned, thus the top (branched, leaved and blossomed portion) gets too big for the spindly trunk. The standard (tree form) is artificial -- manmade by grafting and binding confinement.

There are a dozen or so commonly available cultivars of this impressive species but the old-time variety 'PeeGee' is now much harder to find in commerce. The previously mentioned dwarfs allow one to use this otherwise potentially large shrub (shree, see Addendum under Book) in locations with limited space, but the dwarfs will still require regular pruning to keep in check -- again, no woody plants stops . . .. My preferred paniculate hydrangea cultivars are full-sized. I especially like and recommend 'Limelight' and 'Quick Fire' but 'Renhy' (Vanilla Strawberry) is my favorite. To further illustrate the commonness of false information, I received in today's mail (early January) a spring plant catalog which states that Vanilla Strawberry "spreads to 5" wide" and "Ht. 6-7." I have already dispatched the height falsehood. As for width, my decade old specimen, which I size manage by annual pruning, is still 12 feet wide (before pruning). Question: How can a woody perennial that can produce three (or more) feet of new growth every year (L to R, front to back and high) be less than six feet wide? Answer: Poor growing conditions and/or it is a fabricated claim (lying) to enhance potential sales. And, since most consumers/people are in the trusting/gullible camp, we swallow. The PR (marketing) agrees with what we want to hear and believe, so it must be true (confirmation bias). Why would the nice salesperson or the label ever provide untrue info and, two, why would we/you believe it? The answers start with M and N and ends with MONEY (to get your's) and NAIVETE.

I will close with some additional general paniculate hydrangea comments. Under ideal conditions . . . still air, high humidity, sunny and full anthesis . . . this visually stunning, shallow rooted, easily grown, potentially large and long performing species is also somewhat fragrant (1-2 on a 10 scale), but do not believe any sources touting it as noticeably so -- same with most daylilies. You probably will need to stick your nose close to or in the panicle. And, unlike several hydrangea species, H. paniculata is not as water sensitive (i.e., while it will tolerate lots of water, if the drainage is good, it does not show stress if dry condition prevail). The species/specimens like full sun but, if only unidirectional the specimen can become misshaped (i.e., they will favor the sunny side) what I call photodysmorphia. Furthermore, Vanilla Strawberry is the last of my paniculate hydrangea cultivars to perform -- seems to be a week or so behind the nearby 'Quick Fire' and usually starting the last week in June in Indianapolis. Heavy pruning allows me to produce monstrous Vanilla Strawberry panicles -- up to 18 inches long and 10 inches across at the base of the cone. This is one of the few times one has the opportunity to appropriately use the terms ginormous and humongous. The panicles are so big and heavy they can cause the branches to arch/bow downward up to a foot. The weighing is especially so after rain. Knowing this will happen, I remove during pruning any branches extending to within a foot of the ground. And the species is a fast grower so no need to spend the extra money getting a large specimen to start. Finally, the root system is shallow which limits the ability to cultivate beneath specimens. I hand pull most interlopers and apply a blanket of mulch underneath the canopy.

Hydrangea paniculata is a species I highly recommend -- beautiful, low maintenance and mostly problem free. Moreover, it is highly variable by size (according to cultivar and maintenance), general shape (e.g., 'Quick Fire' is more upright than Vanilla Strawberry) as well as according to the type, combo and sequencing of inflorescence pigmentation (from white/cream, light green, pink to burgundy/red). But remember, it is a woody species, thus indeterminate growth, with the potential to get quite large.