Drooping Leaves are a Problem

Much of the USA has had a hot, dry summer this year. Plants burst forth in the spring with good amounts of rainfall, only to be tricked by hot dry weather later. If you are trying to cultivate special plants, be aware of dry spells and try to water at least once per week, deeply.

Wilted leaves on a Burning Bush, Euonymous alatus 'Compacta'

There are some interesting things about the response of plants to dry weather. In fact, they have developed a special hormone to deal with drought. It is called abscisic acid. ABA as it is known to botanists, is produced in the roots when the soil gets hot and dry. It travels up the plant stems in the xylem tubes and gets into the leaves. There it alters the osmotic potential of the cells which keep the stomata open. Stomata are like little mouths in the back of the leaves. The little mouths are normally held open by the plump guard cells so the leaves can breathe gases and perform photosynthesis. When the guard cells soften up in response to ABA, the stomata close and insure no water loss. This is a very clever thing for the stomata to do since most of the rest of the leaf is covered by waxy coverings and is quite waterproof. Anyway, the ABA is a signal that the weather is hot and dry and is a time of stress, so shut down everything and wilt. When water is again present, the ABA dissolves away and causes no harm.

A large Rodgersia spp. wilted down from drought

Another thing ABA does is prepare plants for dormancy. ABA is produced seasonally at the end of summer in response to the stimulation of shortening daylengths. Different species of plants are more or less sensitive to ABA, so it is one of many factors involved, but the bottom line is it makes the leaves fall off. That is why it is called abscisic acid, it makes leaves abscise. Some species of plants will shed a part of their leaves after a single wilting event. Some plants will not shed them until shortening days, cold weather, and other signals tell them it is time for winter dormancy. In many plants, ABA will stimulate the growth of corky cells located at the base of leaf stems. These corky cells sever the connection between the twig and the leaf. After no more water or nutrients are coming INTO the leaf, and no more carbohydrates from the leaf are going INTO the twig, they feel they can do without one another. The leaf begins to starve, and it loses first its chlorophyll, then its carotene, then its will to live and lets go. That is why leaves turn from green to orangy red (in some species) as they go into autumn. It is getting to that time of year, people…fall.

Apical Dominance

This is the time of year to go outside and check on how your plants are doing. One thing that is super easy to do and will give lots of return for hardly any input is nipping off the tip of terminal shoots. It is so easy to do and will bring bushier plants and more flowers quite quickly.

Plants have a growing tip that contains a bunch of cells called the meristem. One location of meristematic cells is in the growing tip of the shoot. As the meristem differentiates and turns into more cells behind it, the tip grows forward. Along with growth, the area of the tip is the source of production of a whole class of plant hormones called the auxin group. There are many different types of auxins and they have many effects on the plant, but today we are learning about apical dominance.

The apex of the plant is the tip of the growing shoot. At the time of seed germination, the plant has to grow a stem and some leaves and start photosynthesizing. It must send up a shoot, first thing, to get everything going. As the shoot grows, a matching set of supportive roots must grow. Auxin is the hormone which tells the shoots to carry=on and extend, and the roots to get growing and proliferating. Auxin says, “I am sending out this shoot and I need lots of roots to feed it.”

Auxin also suppresses the growth of buds along the stem behind it. Plants make different amounts of auxin depending on their growth habit. A plant that grows mostly as a vine produces lots of auxin which suppresses the growth of branches and buds below it and allows it to elongate into one long growing shoot which we call a vine. You notice that vines are vine-shaped because they do not have a lot of branches, just lots of long terminal growth.

Vines are the extreme of the plant world when it comes to apical dominance. They stretch out and grow few growing points and each tip produces lots of auxin. If you nip off the growing shoot, the source of the auxin production is gone and the suppression of the lower-down side shoots and buds is gone. When suppression is gone, many growing points start to form and grow and the plant takes on a bushier nature.

All plants produce auxin at the terminal point so pinching that off will stop the growth of the branch you nip and allow side buds to grow. Since most plants produce flowers and fruit from the side branches, you can make a plant grow more compactly and flower more by pruning the tips.

But what about the root growth, you say, doesn’t the lack of auxin reduce the growth of roots? Yes, it will a little bit. You should wait for the plant to become established and medium sized before doing much pinching of apices. After it is growing well, it can handle the pinching and will do fine.

After a pinch two shoots will grow and the plant will be twice as bushy.

House Plants Outdoors?

Sometimes it is tempting to drag your houseplants outside for the summer.  You think that the sun will be good for them or the rain will make them happy.  I would not recommend it.  There are a few reasons why this may do more harm than good. 

The most important reason houseplants may be damaged outside is that the leaves that have grown inside have just the right amount of chlorophyll, waxy outer layer and toughness to live inside your house.  The leaves grown inside are usually large, soft and not too covered with outer wax.  They have set up the proper amount of chlorophyll or other pigments to protect themselves from UV rays in the house.  They have not been subjected to wind and drying.

If you move them outside, those leaves do not have much chance against UV rays and lots of sun.  They are not robust enough to withstand the tossing wind and elements.  Most of the existing leaves will be damaged or fall off.  Then the plant will have to force out new growth.  That new growth is acclimated to outside conditions and will do well for the short term during summer.  Of course it usually takes about two months or more to go through the loss of the old leaves, being bare and messed up, and then growing some new sprouts of properly conditioned leaves.  About that time, some bug or pest will find them and start messing them up again.  If the leaves do survive to this point, you might think, wow, my plants are looking like they are coming back nicely. Not so fast.  Now the summer dwindles down into early autumn and you have to bring them back inside before frost. 

Once inside again, they have the same acclimation problem in reverse.  Those leaves that have been conditioned outside to receive increased amounts of lights will not be happy inside.  Some species of plants like a Weeping Fig, will even drop most of their leaves on the floor in response to a changed environment.  The existing leaves cannot deal with the low levels of light inside, so they think they will go dormant and wait for spring.  They have to sprout out more leaves that are used to the inside shade. 

So, the best thing to do is to leave them in their normal positions inside the house.  Just keep them as houseplants.  They will do better if they do not have to go through all that leaf damage and shedding.  No outdoor pests will get on them and travel back inside with them in the fall.  The watering will be more even and the temperatures will keep them nice all year round.

Freeze Protection with Irrigation

I have been asked by one of my avid readers to explain why agricultural crops are often protected from freezing weather by spraying them with water.  That is a good question since if you were to spray a warm body, like a person or animal , with water, they would be very cold.  Plants are not warm bodies and are not affected by windchill in the classical sense. 

First, I must say that there are many factors that go into making a plant winter hardy.  The rootstock, the variety of fruit, the nutrient level of the plant the previous summer, the weather prior to the freeze, the fruit load,  the soil type and the topography of the plantation all impact cold hardiness. 

Assuming all other facters are equal, what is the deal with spraying water all over them?  The short answer is the heat of fusion.  When water changes phases from a liquid to a solid, it emits a burst of energy as heat.  As I recall, it is very small, but something like 2-3 calories per gram of water freezing is emitted.  That burst of heat, of course melts the ice and turns it minutely back into water liquid.  If a liitle drop of water begins to freeze, emits it’s heat, and cold continues to be around it, of course, the cold overcomes the tiny little heat and just freezes it.  In other words, there is a little bit of resistance on water’s part to turning phses and becoming ice.  If all the water on the trees were just frozen, the little heat would be dissipated and the tree would freeze.   If, however, you keep putting water on the tree from an irrigation sprinkler, there is always liquid water coming in.  By its definition, liquid water must be warmer than 32 degrees, plus, it keeps making that heat of fusion burst a few calories to keep it on the knife edge. 

It seems like this would be very minimal and it would not really work that well.  It is true, we are talking saving the trees a few degrees in order to just barely keep them alive and not frozen.  A citrus tree, with all its internal sugars and juices will exprience freeze damage if it is exposed to 28 degrees for more than 4 hours. This is without the irrigation water on it.   So if the water can keep the treess to a minimal level of warmth, it will save the trees.

There is another aspect of irrigating to preserve trees, and that is evaporation.  This is usually a minor point.  Imagine if you sprayed water on a warm body like a human and set that person outside.  The evaporation of the water would cool him even further.  So in order to not have the heat loss of evaporation, there must be about 100 percent humidity.   The math behind it is this.  You must have 7.5 gallons of water freezing and changing phases to produce the heat necessary to counteract evaporation i n a normal situation, say 75% humidity.  That is usually easily done since the method of adding water to the trees is by overhead irrigation and you can pour lots of water on them.  I just mention this because in certain weather conditions such as low humidity, wind, etc. it can become a factor.  Usually by the time all this overhead water pours down, the humidity near the tree’s branches is near 100% anyway.

It is a myth that the ice you see covering trees in the new’s pictures is serving as an insulator and keeping the buds warm.  Ice is a very poor insulator.  Although I think it would block evaporation.  heh-heh

I hope  I will not see any examples of this on my upcoming trip to Florida. Right now I have no pictures to show you.  If you want to read more about this, try this link:

http://edis.ifas.ufl.edu/ch182

Manure’s Disease

I am posting this information because I keep hearing about this condition.  It is actually spelled Menieres disease.  The best description is a few brief sentences from Wikipedia.

Ménière’s disease (pronounced /meɪnˈjɛərz/[1]) is a disorder of the inner ear that can affect hearing and balance. It is characterized by episodes of dizziness and tinnitus and progressive hearing loss, usually in one ear. It is caused by lymphatic channel dilation,[2] affecting the drainage of endolymph. It is named after the French physician Prosper Ménière, who first reported that vertigo was caused by inner ear disorders in an article published in 1861.”

Just as when you hear a new word, you start noticing it all over the place, I keep hearing about cases of this.  I think this is because the symptoms are somewhat vague and occur frequently alone.  When vertigo, hearing loss, tinnitus and a feeling of fullness in the ears all occur together, it is probably a diagnosis one should discuss with one’s physician.  I think I keep hearing about this because of the company I keep.  As I age, my contemporaries are starting to get these problems that plague the elderly.  I also believe one of the causes of fluid retention in the middle ears is high blood pressure and eating too much salt.  We eat sodium in many products.  Try it for yourself.  Look at food labels and see if you are consuming too much sodium in your diet.  Try to get less than 2000 mg.  If you have blood pressure problems or symptoms of Menieres disease, some doctors recommend as little as 400 mg.  It would be darn near impossible to keep to that restrictive a diet, but we all could do better by some awareness and caution with sodium.

Witches Broom Condition

This time of year, I drive around looking at all the trees with their leaves off and I study the branching structure of them.  One common condition that exists is Witches Broom.  It is considered a disease, but I call it a condition since I think of diseases having a pathogenic organism creating them.  The Witches Broom I am referring to is caused by salt.  Here are a few pictures.

Tufted growth from highway salt spray

These photos were taken along the side of a roadway where salt is used to melt ice in winter.  The cars spray up brine and it coats the trees all winter.

Tufted growth habit

Many buds grow with no distance between them

 The salt damages the terminal bud on the branch.  That bud, in usual situations, subordinates the proximal buds by the production of the plant hormone Auxin.  Auxin is a group of hormones with numerous functions.  The function here being the production of a strong terminal growing bud that extends the length of a healthy branch each year.  Without a healthy terminal bud producing Auxins, the lateral buds break and grow into a disorganized tuft of small branches called Witches Broom.  There is no cure for this except for pruning out the tuft.  If no more salt spray occurs, a bud will start a new elongating branch and will grow more or less normally.   

Given the salt spray situation these young trees grow in, they are going to grow little each year and have a stunted habit.