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When is the RIGHT time to thin?

forestry harvesting management thinning timber harvesting Feb 06, 2023

"Are my trees ready to thin?"

A very common question, and rightly so! 

If you've been in the management game for a while (or Googling for a while at least) you've heard how important it is to "time" your thinning appropriately to maximize its purpose and results. 

But WHEN do you actually NEED to thin?? 

You'll hear to start considering it around age 12 or 13, but then you'll also see folks around the neighborhood not thin until 17! 

So WHEN is it? When is the RIGHT time?! 

And if you've been following me for any amount of time, you probably already know my next words... 

It depends! 

There are a variety of reasons why I can't tell you what age exactly to thin at: the growth of your trees, your objectives and the overall acreage and management plan for example play a role. 

But I will provide you with FOUR indicators to look for on your land. 

#1. Shaded out understory.

This will actually happen before you will be capable of thinning, but it is usually the first indicator to start paying attention! Although your understory may be shaded out around age 8, remember your trees must also be of merchantable size --> aka, they must be tall AND large enough to go to market. 

#2. Tree canopy is less than HALF the total tree height. 

The canopy, aka the parts of the tree with live, green needles/branches, should be less than the TOTAL height of the trees, aka there should be MORE trunk showing than branched needles. You can look at this indicator tree by tree, but your indication should be across the stand, on average. This shows you that the trees are now officially tall enough for market, and with the crowns starting to "thin out", it means the area is getting overcrowded for the current tree density. 

There is a red flag indicator here too though: if your canopy gets less than 1/3 of your total tree... it's a big warning sign you're pushing PAST DUE for thinning. 

#3. Basal area is at least 120 ft2 or higher. 

Honestly, this isn't one I expect you to find out yourself, but rather when a forester checks on your stand to determine thinning recommendations, they will likely take a basal area measurement and inform you what it is, along with their recommendations. It is a general forester rule of thumb that basal areas of 120 ft2 are when stands are getting crowded, and it is time to thin (or at least consider thinning out trees). You now ALSO know that number parameter and can understand more confidently their recommendation to move forward with harvesting. 

#4. You have trees dying out. 

This is actually an indicator you are past due (or very close to missing your window of opportunity) for thinning and receiving results. When trees aren't intentionally thinned out, the forest will thin itself to create those gaps needed for continued growth. "Nature is doing what nature is doing... why is this a bad thing?" First, you're literally losing money on the ground... but it is also increasing the overall forest stress of your stand, which increases the risk for insect and disease problems as well as increasing the wildfire intensity potential. 

So, based on these indicators I just shared with you... when is the best time to thin?

It still depends, but generally if you fall within what I will call "the sweet spot" of all three characteristics:
            - Shaded out understory
            - Tree canopy at 40% of total tree height
            - Basal area at 120 ft2

Then I would say you are within your 1–2-year window for optimal results to thin your stand! 

It is typically expressed to thin between the ages of 12-15 years old because that is when it is most likely to reach those indications, which will provide you the most ideal results from your thinning operations. 

Can you thin before or after that window? 

Sure... but it won't provide the same impact. 

Thinning too soon: your trees are probably too small for market but ignoring that they're more likely to have an increased susceptibility to wind damage due to their size and not going to receive the jump in growth desired. 

Thinning too late: the trees may actually be stunted and not respond well, or at all, to the operations, and continue to grow in height while staying skinny chopsticks. 

So next time you think you might be getting close to thin, take a quick look at the indications I shared with you and give your local forester a call to get a professional opinion! 


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