Trimming Your Asters

I learned awhile back about clipping asters from a neighbor who's own asters always seemed to be completely under her control. I wouldn't have ever known she was trimming them, if I hadn't spied her cutting them one day. So, if you want your asters to a) be a little shorter, b) bloom (even) more profusely, and c) be somewhat fuller, then pruning/clipping/trimming (whichever you wish to call it) is the right approach.

Can I prune all asters? With roughly 32,000 species in the greater Family Asteraceae, from dandelions to zinnia, I wouldn't willy-nilly prune without a little research. Some will work with you (Chrysanthemums) and some are best left alone (Lion's Foot). Also, many cultivars of popular asters, have been bread to be dwarf, like Symphyotrichum novae-angliae 'Purple Dome.' It's not worth clipping these. 

 

It's best to stick with the sun loving, taller growing, autumn blooming asters like New England, Symphyotrichum novae-angliae, New York, Symphyotrichum novi-belgii, Aromatic Aster, Symphyotrichum oblongifolium, Smooth Blue Aster, Symphyotrichum laeve, and a few part shade species such as Short's Aster, Symphyotrichum shortii, and Drummon's Aster, Symphyotrichum drummondii. Few would argue that these species aren't already floriferous, so the primary goal of clipping these would be to reduce height by directing energy into growing side shoots of below the cut stems. Reducing height can also, but not always, help reduce leaning. 

The process of trimming/pruning/clipping aster species is similar to the process used by "mum" growers to create giant balls of blooms we see every September. As I mentioned before, mums are in the aster family, and are a good candidate for trimming. Mums are generally not tall plants, however, so clipping stems tends to slightly shorter plants that produce more, somewhat later blooms.

 

How Does This Work?

Clipping promotes more flowering in a healthy plant because its leading vegetative stem growth (apical meristem) is dominant. This dominance inhibits new stem growth from nodes (the places where additional stem growth could emerge) below the dominant stem's growth tip. A plant can have several dominant stems emerging from the crown at the base of the plant. These dominant stems will be responsible for developing inflorescence meristems which go on to produce floral meristems which then produce the flowers we want. 

 

However, if any dominant growth tip is severed, by a deer getting a meal or by your pruner, it becomes a free-for-all among the nodes below the fallen king! In a fairly short time, new stems will emerge from nodes below the cut. These new, "auxiliary" stems are known as axillary meristems or auxiliary shoot meristems. Each of these are capable of producing its own inflorescence meristem, which goes on to produce a floral meristem, which produces flowers. 

 

Now you know how pruning works to produce bushier plants with more flowering buds! By directing plant energy into growing multiple stems, instead of one, pruning this way can keep aster plants a bit shorter and with that, a little less likely to flop when heavy with flowers. The last thing we need to consider is the timing of that cut.

If you cut too late, the plant might not make it to flowering before frost (although many asters are quite tolerant of some frost). If you cut it too early, you may not see shorter plants, or worse, create a leggy plant. The apically dominant stem tends to be thicker and tougher than the auxiliary stems that grow after a pruning. If auxiliary stems, which tend to be thinner, begin growing very early, they will lead to long, thin flowering stems which can lead to dreaded floppy. So trimming your asters is a Goldilocks affair —it has to be (mostly) just right.

 

Timing

It is good to begin thinking about pruning before the Summer Solstice, June twenty-something. Now, it could be two weeks earlier or a week later (July 4 is a good cut off date). If you are in South Carolina, it may be earlier or, if in Massachusetts, it could be on the later side. If the season is warmer, earlier than normal, no matter your location, you might prune a little earlier. Get it? There is some flexibility in your timing —some of which is dependent on your location and some dependent on the seasonal variability. 

 

An important thing to understand is that plants in Maine or Minnesota, in June, are growing considerably faster than plants with a longer growing season, like in Tennessee or Texas. In Minneapolis, MN, on June 21, days are over 15-1/2 hours long and in Houston, TX, days are just over 14 hours long. It might not seem like much, but those plants in Minnesota got a later start and know they've got a lot of growing to do! Point is, keep this in mind when experimenting with the best time to clip —and keep mental or paper notes.

How Much To Clip

The aster species I mentioned above grow from three to five feet, depending on quantity of rainfall, soil moisture, etc. I expect the earlier blooming, New England Asters to be roughly 20-24 inches by mid-June. I might want to wait a little longer, until they've reached 28 inches and cut them back to 22-24 inches. New York or Aromatic Asters, bloom later and grow shorter than New England Asters. Therefore, I may cut these back to 18-20 inches when they've reached 24 inches. There's no hard and fast rule I can give —you'll need to experiment with an eye toward cutting the aster back 15-20 percent at pruning time. 

Asters, like these, are tough and are difficult to harm. The simple rule is: do not cut too late and do not cut too much. No later than July 4th with about 20% trimmed off is my best general guidance.

 

There are over 40 years between our Felco #2, bottom & Felco #9, top, for lefties.

 Tools of the Trim

There's more than one way to clip the early, soft-stemmed growth of an aster. Here's a list:

• Pinch and snap between two fingers is the cheapest way.
• Bulk trimming with scissor shears, like these sharp Felco #250
  
• Clip with the best set of pruners, like Felco #2 or Felco #9 (for lefty)
• If you are cutting a border or edge full of asters, you could scythe:
• I inherited two scythe: seriously sharp and you feel sharp:
• Muller Scythe Blade from Austria and Muller Snath (handle)
• I mean, why gas up a weed wacker when you can silently scythe?
  

This is Smooth Aster, clipped somewhere between 20 and 24 inches.

New York Aster clipped about 16 to 18 inches.

In more shade, this New York Aster grows slower, so is clipped later than most.

To see what to do with your asters once they have flowered in fall, see this post.