Up and Down and All Around

A customer of mine who lives on a beautiful hilly and rocky property on Forest Lake has a problem – a beaver problem. Early last year the furry critters girdled two 16 inch diameter sweet gums on the back hill. They took off about two feet of bark and underlying green tissue all the way around each trunk.

Now, as you know, plants manufacture food in the leaves where photosynthesis takes place. This food is primarily in the form of sugars and is distributed to the rest of the plant where it is used as a source of energy in the production of the many complex components of the plant such as fats and proteins and DNA.

This movement of this energy producing food takes place in specialized tissues called phloem. In woody plants like trees the phloem is just under the bark extending in an unbroken network from leaves to the tips of stems to branches truck and root. In this tissue flows the lifeblood of the plant.

And that is bad news for our two sweet gum trees. Because of the break in the flow of this food source to the roots of the tree, in time these roots will die. When the roots die the tree will no longer be able to get any water, which means the tree will die.

But oddly, 18 months later the tree is not dead – indeed, it has a full canopy of leaves! How can this be?

Plants, thankfully, are adept at storing food for future use. Seeds are so nutritious for animals because the plants have stored so much concentrated food there for the growing seedling. Many plants store food in underground stems or tubers or bulbs. Most plants store food in the tissues of roots and/or underground stems.

The girdled sweet gums had obviously stored a large amount of food in their roots because those roots are still alive producing active root hairs and transporting water and nutrients through the xylem tissue deeper in the trunk to all the limbs and shoots and leaves above. But this food is going to run out at some point.

Today I was at another customer’s house. A couple of months ago we decided to cut back two old and very large chindo viburnums to about a foot in height, and let them start over. It took a while for the stumps to put out new growth but today I noticed that there was an abundance of rich green shoots about two feet high.

In the case of the viburnum we had cut off all means of the plant manufacturing its food via photosynthesis. There would be no sugars transported from leaves to the roots. But thankfully, again, the roots had stored an ample amount of food to supply new buds and tiny new leaves with all the food they needed until they are able to produce enough to begin feeding those same roots again.

How new growth is activated in a cut stem or trunk is fascinating – but for another article.

So imagine that every time new growth appears we cut it back. This could go on for some time but eventually it will stop. The plant will have used up all its food reserves in the roots, and it will die.

This sounds kind of sad for a once grand chindo viburnum, but what about for wisteria? In that case it sounds like a plan.

It is commonly thought that wisteria is nearly impossible to eradicate. Cut it back and new shoots pop up not just at the main stump from underground runners that seem to be everywhere.

Well, grab a sharp set of hand pruners and once a week take a few minutes and just cut back all the green shoots and leaves you can see. After a while there are fewer and fewer of them. Eventually they stop altogether. You have starved the wisteria to death.

The fact is that the complex transport and storage of nutrients in plants is utterly necessary to plant life. It happens in different ways in different seasons. In summer, where we in Columbia find ourselves now in that drama of an earth spinning along a tilted axis in its annual trip around the sun, food is being produced at a maximum rate by mature leaves and sent to all parts of the plant.

The very newest leaves at the tips of the stems are not self supporting yet and so food is sent to them to help them grow and get started photosynthesizing on their own.

Flower buds require a huge amount of food. In fact perhaps the most important movement of food resources is to the reproductive tissues of the buds, flowers, and fruits all of which exist to produce and protect and enhance the spread of the seeds! For in the seed is new life, and new combinations of DNA to keep the species flexible and healthy over the course of time.

Today I was noticing the seed pod of a camellia at the same place as the chindo viburnum. This seed pod was maybe an inch and a quarter across. It has not yet fully matured.  I tried cutting it open with my hand pruners but they didn’t really open wide enough to get a good cut. When I was a kid following my grandmother around her garden in the fall she would grab mature camellia seed pods that were ready to open and hand me the seeds. I planted many of them back at my house. But Nanny was not fond of those fruits and seeds. She explained to me that all the energy that went into growing those fruit pods and seeds was energy not going into flower buds. She grew camellias for the flowers not the seeds, and if she wanted new ones that she liked she rooted cuttings. One never knows what kind or color camellia will emerge from a seed.

But the lesson was that we can impact the growth of different parts of the plant by impacting how food energy is dispersed. If you grow knock out roses for example you will see that after the flower blooms it grows a fruiting body called a “hip.” Well it is likely that you are not growing knock out roses for the rose hips but for the flowers, and if you want more flowers you need to keep those hips from forming. Thus you pinch off the spent flowers, called deadheading. With many plants this encourages a new round of flowering. Chaste trees are a good example – if you snip off those spent clusters of flowers you may be rewarded with a second flush of blossoms in late summer.

The production of over wintering flowering buds also requires a large amount of plant energy. For a plant like camellia too many flower buds mean flowers that are too small! So if you are growing camellias for flower cuttings, fewer stems and fewer buds mean bigger flowers.

As summer ends those highly productive leaves (at least on deciduous plants) begin a process of planned death and use much of their late season energy production to break down larger more complex starches and proteins into simple sugars and amino acids, which are then transported along with important nutrients to areas of storage – the roots, the stems, and the outer bark just outside the phloem of the stems and trunks. There, as the plant enters into winter dormancy, these food reserves are ready when winter turns into spring and a new season of growth begins.

Which gets us back to the beavers. The beavers that girdled those two sweet gum trees were not hoodlums. Nor were they planning to use the 60 foot high trees whenever they would die and fall in their dams. Just just inside the bark of the tree is a layer of cells called the vascular cambium that produces the phloem tissue that carries nutrients around the plant. Not only are the cambium tissue and phloem rich in nutrients, but there are other cells in the inner bark which store food for use in the spring. This rich tissue underneath the bark is a rich food source for beavers which also have the capacity  to digest the hard bark and wood tissue around the phloem. So the beavers girdled the trees because, well, they were hungry!