About Those Plant Sizes

April 24th, 2012

   One question I get all the time that I can never answer very well is, “How big is this plant going to get?”

Accurate size planning heads off this kind of problem.

    Usually it’s a case of people trying to figure out if they have enough space for something. Or they’re trying to avoid house-eating monstrosities.

   But I also get it from frustrated plant-shoppers who have done some homework and found that different growers list different sizes for the exact same plant.

   How can that be? Why can’t everyone agree? And if they can’t, then whom do you believe?

   What it boils down to foremost is how far out you’re using as your guide to estimate size.

   Plants never really stop growing… that is, until the deer eat them or a drought kills them. So you’ll see a different size if one grower or garden center figures on a 5-year size but another uses 10-year (or beyond) sizes.

   I’ve seen a 100-year-old yew bush at Longwood Gardens, for example, that’s easily 25 feet tall and wide. But most people hack yews into 4-by-4 boxes, and most plant tags list them as somewhere close to that range.

   A second complicator is that plants grow at different rates in different settings. I’ve seen the same variety of plant grow to drastically different sizes in different yards.

   Things like soil quality, soil nutrition, soil moisture, light and the temperatures in different microclimates can all make a big difference.

   That explains why one gardener might complain about having to whack back an azalea every year while another gardener says the same azalea in his yard hasn’t gone above 4 feet in 20 years.

   This creates a problem when you’re the one responsible for putting numbers on the plant tags next to “Height” and “Width.”

   The more adventurous types will take a stand and put down, “6 feet tall, 4 feet wide.”

   More conservative types will put down, “4 to 8 feet tall, 4 to 8 feet wide.”

   In the first case, you risk the wrath of gardeners when the plant works its way up to 8 feet tall in, say, 10 years.

   In the second case, you end up with such broad ranges that the numbers don’t help much at all.

One solution: Go by "maintenance size."

    What I usually go by is a good maintenance size. In other words, when a woody plant starts hitting its stride somewhere in the range of 5 to 10 years down the road, what’s a good height and width to keep it at?

   That’s the point where you start pruning it for size-control purposes. Most plants will slow their growth rate as they become more and more mature, but they never grow to a certain size and then just stop.

   It’d be nice if plants had fast-forward and stop buttons, but they don’t (at least not yet).

   Listed sizes are at least usually helpful for determining plant proportions. They’ll tell you if they’re more upright than spreading, for example.

   But don’t take any listed size you see as the gospel truth. Look at tag sizes more as guides or “estimated averages.”

   For what it’s worth, I list good maintenance sizes on all of the hundreds of plants on my “George’s Survivor Plants for Central Pa.” list.

   The list is available as a $5.95 download at https://georgeweigel.net/helpful-info-you-can-buy or you can order a paper copy for $10.95 (includes postage) at the same link.

   Happy planting. This is prime time for getting new trees, shrubs, evergreens and perennials in the ground.