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Pruning Paulownia

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Pruning Objectives & Techniques for Paulownia tomentosa

by David Sutton, October 1995

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I want to give you a better understanding of Paulownia tomentosa and how to modify its growth through pruning. Please understand paulownia’s response to these pruning techniques will depend upon it’s vigor, it’s quantity of stored energy, and its genetics.

There are many growth factors involved in raising paulownia, such as site quality, length of growing season, available moisture, competition, vigor, genetics, and so on. Please realize it is the not the net of all these factors that produces growth! Rather, it is the one most limiting factor that restricts growth from reaching its full potential. It doesn't matter how great all the other factors are. Growth is reflected by the most restrictive agent. When you hear of paulownia's fantastic growth, the limiting factor is probably the tree's genetics.

Tomentosa's Nature:

1. Tomentosa is a very strong sprouter. This means that you can heavily prune and the tree will come back with more sprouts. You can force the tree to grow to your wishes because you can force the tree to develop buds anywhere. I classify 3 types of buds. There are primary, secondary, and tertiary buds. The primary and secondary buds develops at the leaf scar. The primary bud is the strongest and usually develops first. It is the most terminal bud above the leaf scar. The secondary bud develops between the primary bud and the leaf scar. It usually buds after the primary. The tertiary develops elsewhere on the bark. It is the last to develop. Tertiary buds usually develop strongest when the tree is very heavily debud and delimb in early spring.

Please understand tree vigor is important to bud development. A tree with poor vigor will not produce the quantity of buds as one of high vigor.

2. Tomentosa has no true terminal leader. If it has a leader, it is lost at the end of the growing season. The following season, it must develop another terminal leader. Ninety-nine times out of a 100 it will not. No bud will allow another to become terminal and dominate. This makes for poor form.

3. Tomentosa has weak terminal sap flow. After first log extension, tomentosa puts on little terminal growth. You need to think of tomentosa's sap as lazy. It doesn't want to travel upward any more than necessary. If it can grow, develop, and produce wood fiber at ground level, that is where the sap will flow. For height growth, you must heavily delimb and debud to force sap flow to the most terminal end of the log. Buds in this location will not develop and become dominate if there is little sap flow to them.

4. Tomentosa has an inverse relationship between height growth and lateral limb development. You get maximum height growth and minimum diameter growth when there are no lateral limbs, just a terminal leader.

5. Tomentosa has a direct relationship between diameter growth and lateral limb development! You get maximum diameter growth and minimum height growth when all lateral buds are allowed to develop into limbs.

6. Tomentosa's growth is directly determined by crown size. The more foliage the tree has, the more wood fiber it can produce. Any bud or limb removed will reduce crown size. This is going to reduce some from of growth! Also, the frequency of debudding will reduce growth. If you heavily debud for the first 6 weeks of the growing season, you have lost 6 weeks of growth. Lost of growth is O.K. if it would have resulted in undesirable growth.

Pruning Methods

Pruning activities can be categorized into three major groups. You prune for height growth, diameter growth, or form. When you prune, you need to address goals. You need to prune for wound healing, proper tree stiffness, good tree form, and diameter growth rates. When your log quality and length matches it’s site, it is time to start diameter development. Don't prune unless you have a reason. Remember, any time you prune it reduces woody fiber growth!

1. Debudding is a pruning method used to control growth and form. It is the removal of any developing bud before they become woody. It is the simplest and most economical way to keep a clear log and control the tree's growth rate. When forcing height development, debudding begins when buds are first noticed. You want to force the sap's activity up to the most terminal end of your log to promote bud development in that area. Otherwise, you can start debudding after frost damage is over. There is no sense in removing buds when mother nature can do it for you. Remember, the tree can form buds throughout the growing season.

2. Delimbing is a pruning method used to control growth and form. It is the removal of unwanted lateral limbs. It is best to delimb during the dormant season. The limbs must be pruned before they reach 3/4" in diameter. If larger, good wound healing may be difficult and log quality may be sacrificed. In most cases, the unwanted limbs need to be removed after their first growing season. If left for a second season, they may be too large to prune. To minimize wound size, prune at an offset angle, away from the tree's trunk. Don't cut flush to the stem.

3. No coppice is a pruning method used to develop log height without coppicing! It's not recommended, but it is possible to get good straight form without coppicing. It should only be considered where sites are poor and height growth is limited to just a few feet a year. The advantage of this technique is that there is no wound at the base of the tree. Stump wounds make the first log extension weak. It is an open entrance for possible rot. They can take more than two years to heal. No coppice should only be used as a last resort to get height growth when coppicing fails!

4. Low coppice is a pruning method used to develop height growth. It is the most common technique to get your first log extension. It is also used to correct major trunk's defects that are not healing, such as sun scalding. You cut the tree 1'' above the ground, angling the cut to the South. You can expect good log form and attain heights from 8 feet to 25 feet in one year. If you don't achieve at least 8 feet, you will need to consider the no coppice technique or to abandon you plantation in search of a better site! It is a good idea to treat the stump against fungi attack. This will help maintain tree vigor and protect against rot. Lime water is what the literature recommends. You should coppice the tree when the root collar is 2 to 3 inches in diameter or the tree's height is about 8 to 15 feet. After cutting, remove the coppiced tree from its site. This is a good preventive measure against disease and keeps the ground clean for future work. When sprouts develop, pick the one best sprout. Remove all others. You will need to return no later than once every 2 weeks to remove more developing sprouts. Remember, tomentosa is a strong sprouter!

5. High coppice is a pruning method used to develop height growth. It is used after the first log extensions. The terminal end of the log needs to be forked. It is used when you have allowed lateral limb development the previous season to increase diameter growth. You want to increase stem stiffness and root development so that the tree can support new height growth without getting top heavy! High coppice is basically the same as low coppice. The difference is the stem is cut just above the forked terminal. You want to leave bark between the cut fork so that a bud can develop between the fork. The tree needs to be completely delimb before spring budding. You want to get the tree back to a straight form! You want a tertiary bud to develop on the terminal end of the fork. At spring budding, the tree must be heavily debudded at weekly or bimonthly intervals to force sap flow to the terminal end of the log. When a strong terminal leader has developed, all debudding should stop so growth can start. It may take up to 6 weeks to develop this terminal bud.

6. Forced primary bud is a pruning method used to develop height growth. It is used after first log extensions. Here the primary lateral bud is forced into a terminal leader. In this technique, the stem is not coppice. Rather, it is left alone. It is used when a non forking tree already has good form and proper stiffness and is ready for the next log extension. As in High Coppice, the tree must be heavily delimb and debud at weekly or bimonthly intervals until you have forced the most terminal primary and secondary bud to develop. Only allow one side to develop. Only allow 2 buds to develop; one primary and the underlying secondary bud. Other wise, the tree will become forked or there will be no terminal dominance. When the secondary bud has pushed the primary bud up and parallel to the existing log and the primary bud shows terminal dominants, the secondary bud should be cut off about 1" from the trunk.

One warning on bud development. Buds don’t always form where you want them! The more vigor your tree has, the easier it will be to obtain needed buds for proper growth and development. For example, using the high coppice technique, a tree with poor vigor will develop buds, but they may be few and they may not be at the top of the fork! After 4 to 6 weeks, you must stop budding and let the tree grow. When you are stripping the tree of all green growth, the tree is starving for food. If you continue to entirely debud, the tree will die.

Pruning Tools

They should be light and require minimum labor. You need an aluminum pole that can reach to 25 feet, a fine tooth saw, a debudding tool of your design, a pair of hand pruning shears, and a pair of sun glasses. If you can justify the cost, a custom made field ladder should be considered. It becomes difficult to prune effectively past 15 to 20 feet. If you wish to have a 30 foot clear log, you must have a ladder. I am designing one to be attached to my tractor's 3 point hitch.

Plantation Management Pruning

Now that I have discussed tomentosa's nature and pruning methods, you must understand plantation management before you can prune.

I classify 3 distinct phases of plantation management. The first phase is establishment and root development. The second phase is height development. The third phase is diameter development. Tomentosa's growth should be fast and rapid during the first two phases. The third phase is when you are actually growing your board feet for future sales. Your market should determine the growth rate of this third phase. The management of the second and third phase can be based largely upon pruning. Successful growth management is based upon proper pruning of the lateral buds and limbs. You must also match pruning to the site's ability to produce growth. Don't try to grow a 30 foot log when your site can only support a few feet of growth a year. You want to grow a 30 foot log when your site will produce a 20 foot sprout the coppiced year.

As I said, the second phase is height development. Depending on your goals and site, this phase may take from one to 10 years. After the coppice year, you must develop log length through yearly log extensions. You must force height growth through proper pruning techniques; stepping the log up higher each year until your desired height is reached. In the process, you must always maintain proper stem stiffness! You must have adequate diameter growth to support the crown's weight. If you don't, the tree is going to be top heavy and have poor form. The tree will bend over.

Let me talk about the crown to clear trunk relationship. It is a very important concept in tomentosa management. Let's take 2 trees both of which are 40 feet tall. Tree A has a clear trunk of 10 feet, the remaining 30 feet is crown height. Tree B has a 30 foot clear trunk, the remaining 10 feet is crown height. Which tree's trunk will have the fastest diameter growth? The answer is tree A. Not only has tree A got 3 times the foliage to produce wood fiber on the trunk, it only has to put it on 1/3 the length. This is a 9 fold difference! When all growth factors are non limiting, you will see a tremendous diameter growth rate when the ratio is greater then 1/1. This rule (Crown to Clear Trunk ratio) is important to follow when you have gotten your plantation's height develop and are starting to concentrate on diameter growth.

As I said earlier, the third phase is diameter development and board foot production. This is the time to maintain your desired diameter growth rate. Height is no longer a concern. This is the time to keep accurate diameter measurements every 2 to 4 years. You need accurate records to make sound management decisions. If diameter growth is too rapid, you may need to reduce crown size by delimbing. If diameter growth is too slow, you may need to do a partial harvest to release competition. You need accurate records to know your lumber volumes and lumber quality. These records will document the true value of your plantation whenever that information is needed. Record taking does not required a lot of equipment. You need a measuring device such as a tree caliper or a measuring tape. You need a permanent label so that you can identify each tree. You need pencil and paper. Record keeping is very important for good management! Without it, you don't know how to manage the third phase.

A main concern in the third phase is maintaining your predetermined crown/clear trunk ratio. This ratio is based upon the markets you are catering to. A ratio of .25 would be geared for slow diameter growth. It would be managed for the high price Japanese markets. A ratio of 4 would be geared for rapid growth. It would be managed for other specialty markets here in the USA.

You must also have a balanced crown. Not only do the Japanese grade on annual ring tightness, they also grade on the pith being centered! You must have a balance crown to have a log with a centered pith. A balanced crown is one that has the same amount of foliage on all sides of the tree. For example, if the crown has no foliage on the North side and full foliage on the South, East, and West, the pith will be off center, favoring the North. You need a balanced, uniform crown on all sides so that equal amounts of wood fiber will be produced and laid on all sides of the trunk.

Conclusion

In summary, I hope I have given you a better understanding of tomentosa, it's growth habits, and how you can modify growth through pruning.