Beautiful Native Plants

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Wednesday, June 29, 2016

Plants have hormones, too

A topped magnolia (M. grandiflora) has produced
several side branches after the terminal bud was cut.
Don't buy this tree!


The terminal buds of plant stems contain hormones (auxins) that suppress the side shoots from sprouting and also encourage root growth. You’ve probably noticed the result of this hormone when you cut off a shoot: one or two new shoots will grow from the next nodes that were just buds before.

The auxins also help with rooting, so much so, that commercial rooting hormone is a mixture of auxins. We all want our plants to survive and prosper, so here’s what you need to know about these plant hormones.

Don’t buy this tree! 

The magnolia is bushy, but an untopped tree with one true leader (and all its auxins) will be much stronger and will not need corrective pruning to choose a new leader. Also the presence of the terminal bud will encourage faster root formation after planting.

Why do growers do this to trees? For many years the so-called garden experts recommended bushy plants. It's an old gardener's tale. But a topped tree of any size will have a shorter life. Another reason not to buy this tree is that it is too large and has been confined in a pot for too many years. It has a much lower probability of adjusting to your landscape than a small 2-year sapling and if it does survive, it will take a much longer time to begin to grow.

What do hormones mean for gardeners?

There are two gardening activities where knowledge of how auxins work is important for the best success: planting and making cuttings. I’ve written about better planting techniques and ongoing care required for landscapes–native or otherwise–in: Native Plants for your yard: the next step. What I didn’t mention at the time was that one of the reasons behind these better planting methods and ongoing care was the specific role that auxins play.

A) Planting

Unfortunately many people who buy plants for their landscapes will not plant them using the best practices and many plants die. This discouraging result keeps some people from planting more natives and the neighborhood ecosystem loses. So let’s be smarter…

The old gardener’s tale says to prune heavily when planting or transplanting trees or shrubs to reduce the amount of upper structure for the damaged roots to support. This is absolutely the WRONG thing to do for two reasons:
1) removing terminal buds from a plant reduces its ability to form new roots, because removing the terminal buds reduces the available auxins
2) the plant needs all its leaves because they produce sugars for energy for recovery and healing through photosynthesis.

Yes, the roots are damaged, especially if you’re dealing with plants grown in containers, where the best practice is to rinse away all the soil. The remedy for damaged roots is adequate irrigation, not pruning. Irrigating thoroughly at planting time, every day for a time depending upon plant size, and then regularly after that will increase the likelihood of success. I’ve described the recommended irrigation routines in more detail in my previous articles cited above, but the point of all the extra water is to allow the plant to take up enough water to transpire and photosynthesize at normal rates even though its roots are damaged.

This much abused winged elm (Ulmus elata) needed a lot of attention. 
Before planting this elm, the circling roots were straightened out resulting in the need for extended irrigation and no pruning for a full year. The corrective pruning took 3 years to clean up the messy lower branches and to select a main leader from the 3 side sprouts. Prune only 20% of a tree per year. Read more details on this tree in Native Plants for your yard: the next step. (Update: 5 years after planting, this tree had one main leader and was nearly 20 feet tall.)

Here’s what can happen if you don’t rinse away the soil from trees grown in pots. This Kentucky yellowwood (Cladrastis kentukea) has choked itself to death because the soil was not rinsed away. So when you invest in a tree, you also need to invest the time needed to plant it right and then to supply enough irrigation to help it heal its damaged roots.  And don’t prune it for a year or more.  Photo used with permission is by John Manion.

B) Making cuttings

As gardeners we like to increase the sheer number of plants to fill in our landscapes and making cuttings is one way to accomplish this. Be sure to take cuttings from several different individuals for the widest genetic variety in your landscape, because each cutting is a clone of the parent plant. If the species is dioecious, be sure to take cuttings from both male and female plants—especially if they will be planted some distance away from the parent plants.

Since the auxin hormones help to encourage rooting, your cuttings will be more successful if each one includes a terminal bud where there are natural auxins. There are various commercial hormone powders on the market, but you can also make a willow tea for a more natural solution. (See below.) When hormone is applied to the stem, it coaxes the stem tissue to sprout adventitious roots. Some plants readily sprout roots from their stems, but others need the hormone application to root.

The other factor to make your cutting percentages higher is to bury more than one node or bud in the soil. The nodes are growing areas that contain meristem tissues or undifferentiated cells that would normally become leaves or branches, but those cells can also become roots. These are literally stem cells.

Some plants readily form roots when put in water
 like these basil cuttings..

Some plants root readily in water, but water roots have a different structure. When you plant a stem with water roots into soil, those roots will rot away and the stem has to generate all new roots that work in soil. So pre-rooting in water may actually slow down the process of developing “real” roots and is not usually recommended for woody plants. On the other hand, holding your stems in water will keep them turgid until you have a chance to process them properly.

If you are using powdered hormones, prevent possible contamination of the entire supply by putting some in a separate container where you will treat the cuttings. Dip the end of each cutting into the hormone powder until it’s coated on all sides, and then tap the cuttings to remove excess hormone powder. Make a hole in the sterile rooting medium, carefully place the cutting in the hole so the powder doesn’t scrape off, and then press the medium firmly around the shoot so that is stands upright. Don’t put the leftover hormone back into the original container, but you could sprinkle it on the soil surface.

Making and using willow tea

Willows (Salix spp.) root so readily that if you just stick branches into the ground, 80% will root and grow. You can extract the natural willow hormone (indolebutyric acid, a specific type of auxin) and use it to aid in the rooting of your cuttings. Harvest the young wood that still has green bark–early in the year is best. Cut the shoots into one-inch pieces, bruise or mash the twigs to extract more of the hormone, place them in a mason jar until half full, fill the jar with boiling water, and then let it sit uncovered overnight. Keep your willow water refrigerated, but use within a few days.

There are two ways to use this to help your cuttings form roots:
1) Pour two or three inches of willow water into smaller jars, put your cuttings in the water like a bouquet, and then let them soak up the water for several hours or overnight before placing them in the rooting medium. It’s best not to mix species in these small jars because they could react differently.
2) Arrange the cuttings in the moist rooting medium so they stand up. Use willow water for the first two or three irrigation sessions.

Some plants root readily and form adventitious roots from their stems. This beggar tick (Bidens alba) stem was laying on the ground and so was ready to take root. No rooting hormone is needed for this vigorous plant, but others may need more coaxing.

Be a more successful gardener

So now that you know something about auxins, go forth and multiply your plants.

© Ginny Stibolt