Drying Fast-Growth Hybrid Poplar

Advice on how to address cupping of hybrid poplar during air-drying and kiln-drying. January 29, 2009

Question
I have been sawing and drying for use in cabinets. The trees were planted for chips but never used. They are from 12- 20 in dbh and were flat sawn 1 1/8" air dried this spring 16 inch sticker spacing and weight on top. But almost every board cupped, up to 1/4 " on a 6-8 " board. I had some kiln dried and they were the worst. Is it the trees or me?

Forum Responses
(Sawing and Drying Forum)
From contributor S:
If you have any more of the big logs you can try to quarter saw them for better results.


From the original questioner:
I tried 1/4 sawing and threw away most for side bend. The trees are about sixteen years old and grew fast except the last four years. I would like to figure it out as I have sawn about 5,000 bdft and have about 12,000 still standing. I am able to use the lumber by ripping down but would like to dry flatter lumber to sell.

From contributor S:
In the end, perhaps feeding the wood into the chipper is the best thing to do.
Regular poplar is pretty cheap (around here in PA) so I'm wondering why this hybrid poplar is worth the hassle.


From contributor Z:
When you quartersaw lumber if you leave the pith on the edge of the board you will usually get "side bend". Try ripping out about 1.5" away from the pith and toss.

From the original questioner:
The trees are free so rather than pile and burn just trying to use the wood instead of putting smoke in the air. Even with ripping it makes good drawer and frame stock for cabinets, would just like to get it to dry flatter. I have my own mill and kiln so the cost is not the issue. I am getting all clear wood so it’s worth trying to use.

From contributor D:
You may find try leaving them in thicker cants like 12 x 12" to dry longer before the thin boards are cut from them. The reason is in the growth rings that are 1/4 inch wide spacing compared to oak 1/16 spacing.

This may help reinforce it from twisting and cupping as it dries. The reason is that between the growth rings is like sponge fibers full of water that shrink very fast from the fast rate that it has grown which results in shrinkage at a very high rate. Seal the end grain several times to help slow it down. Also, if the tree was standing out in the open sometimes results in a twisting growth and you can't help that.


From Gene Wengert, forum technical advisor:
Excessive warp is a characteristic of plantation grown hardwoods. Rapid kiln drying is the best way to control warp, along with top weights (over 100 lbs per sq ft). High temperature drying also works but is not practical for most hardwood uses except dimension stock.

Cupping is more common when the lumber is flatsawn from near the center of the tree (first 15 to 20 years of growth).


From contributor T:
They are very fast growing trees with wide growth rings and a lot of movement when they dry. I cut them large, let them dry, plane out the cup, and use them for backs, sides and even paneling. The light color is sometimes nice in a room.

From contributor F:
I have found that the slower you can air dry the material the better the results. I place all of my lumber to be kiln dried in a heavily shaded stand of douglas fir tree, to let air dry at least 90 days before the kiln introduction.

From Gene Wengert, forum technical advisor:
With this wood and with many woods, slower drying (high RH) actually produces more warp than fast drying. Technically, with low RH, you develop a lot of tension set and a dry outside shell that is stronger and better resists the tendency to warp.

With hybrid poplar, you have a great deal of internal stress, tensionwood, SOG, longitudinal movement and small diameter logs, that all result in a high tendency to cup and sidebend, so it is not like douglas fir or many other species that are commonly handled.

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