Gardening Study School: Factors Affecting Plant Growth Outline

Lettuce covered in frost looking
decidedly other worldly. Want to bet
it's growth was slowed in this situation?

A.  Growth defined

The progressive development of an organism; an irreversible increase in volume due to the division and enlargement of of cells. The processes of growth, including the three major functions that are basic to plant growth and development

1. Photosynthesis – The process of capturing light energy and converting it to sugar energy, in the presence of chlorophyll using carbon dioxide and water.
2. Respiration – The process of metabolizing (burning) sugars to yield energy for growth, reproduction, and other life processes.
3. Transpiration – The loss of water vapor through the stomata of leaves.

There are two sets of of factors that affect plant growth: Environmental and Genetic

Each can be a limiting factor in plant growth. These environmental factors do not act independently example - inverse relationship between soil moisture and air

B. Environmental Factors                                                 

      1.  Light – variations, intensity, duration

The nature of light

Light changes through the seasons

Quality, intensity and duration of light are important

1. Quality can't be controlled on a field scale - Feasible on specialty crops

2. Intensity of light (brightness) is an important factor.

photosynthesis light intensity

3. Duration - Photoperiodism - Plant behavior in relation to day length

- long day plants - flower only if days are longer than same critical period - 12 hours Grains and clovers

- short day plants - flower only if days are shorter than a critical period soybeans.

- indeterminate - flower over a wide range of day lengths. Tomato, cotton, buckwheat

Plants compete with other plants for available light, nutrients and water – weeds are invariably plants that can out compete our preferred plants. Weeds grow faster, taller blocking out light or can access water and or nutrients better than our desired plants; that's why we remove them.

      2.  Water/Moisture

Availability and lack of water effects

Wilting point and permanent wilting point

Water and roots

Water and leaves

Water in the xylem and phloem

Plant growth restricted by low and high levels of soil moisture

1. can be regulated with drainage and irrigation

2. good soil moisture improves nutrient uptake

If moisture is a limiting factor fertilizer is not used efficiently.

      3.  Temperature

Metabolic changes

Nutrients in cold/warm soils

Most plant growth occurs in a fairly narrow range - 60 - 100 degrees F

1. Temperature directly affects

photosynthesis (slows)

respiration

transpiration - loss of water

absorption of water and nutrients

2. The rate of these processes increases with an increase in temperature responses are different with different crops

cotton vs collards or potatoes

fescue vs bermuda grass

These generalizations hold within a crops range of adaptation

3. Temperature also affects soil organisms: nitrifying bacteria inhibited by low temperature. pH may decrease in summer due to activities of microorganisms

4. Soil temperature affects water and nutrient uptake

High temperatures cause increased respiration sometimes above the rate of photosynthesis. This means that the products of photosynthesis are being used more rapidly than they are being produced. For growth to be sustained photosynthesis must be greater than respiration.

      4.  Atmospheric conditions; effects of pollution

Carbon dioxide makes up 0.03 per cent of air by volume. Photosynthesis converts carbon dioxide to organic material in the plant. Carbon dioxide is returned to atmosphere by respiration and decomposition

In a corn field or closed greenhouse CO2 level may drop and become a limiting factor in growth.

Increasing CO2 can increase crop yields respiration of plants and animals - decomposition of manure or plant residue may release CO2

Adding it to a greenhouse at very high concentrations for several hours will eliminate pests.

greenhouse crops

Plant growth and quality can be enhanced by supplemental CO2. Growth responses have been shown with tomatoes, lettuce, cucumbers, flower crops, greens, peas, beans, potatoes

Air Quality

Air pollutants in sufficient quantities are toxic to plants sulfur dioxide - provides sulfur at low levels

Acid rain – acidifies water has demonstrably killed trees

Thinning ozone – permits ultraviolet radiation harming plant growth especially in seedlings UV radiation is intimately tied with genetic mutations

Particulate matter in atmosphere reduces photosynthesis and clogs stomata on plant leaves also inhibiting photosynthesis

      5.  Nutrients

the 18 nutrients

Nutrients from the atmosphere:

Carbon, hydrogen and oxygen

Primary nutrients:

Nitrogen, Phosphorous, Potassium (NPK)

Minor nutrients - they are minor only in that not that much is required by the plant, still all are essential to plant growth

Calcium, Magnesium, Iron, Boron, Manganese, Copper, Sulphur, Molybdenum, Zinc, Choline

Justus von Liebig (a German chemist) considered the father of modern chemical agriculture

Leibig gave us NPK – the three primary minerals for plant growth

Liebig's Law of the Minimum

How plants take up nutrients

Most of the nutrients used by the plants are absorbed by the roots from a soil solution

Nutrients in the soil

Nutrients in the plant

Over fertilizing causes burning and leaves; possible collapse (death) of the plant and is a principle cause of polluted ground water. Furthermore the lush growth promoted by over fertilizing is delightful eating for most insect pests.

C.  Genetic Factors (Heredity)

Yield potential is determined by genes of the plant. Characteristics such as quality, disease resistance, drought hardiness are determined by the genetic makeup.

Ornamental plants - not interested in total growth as much as appearance

control over the genetic factor by his choice of variety.

Field crops - highest yielding, disease resistant, etc.

Nursery - Best appearance - dwarf vs larger shrubs

Inheritance in plants

F₁ generation and beyond 

Gardening Study School: Pruning Handout

On both these illustrations, you can clearly see
the Bark Branch Ridge and where to cut
in relationship to it.

General Notes:

Pruning involves the selective removal of specific plant parts for the benefit of the entire plant.

• usually involves the removal of branches
• can include removal of roots, seed pods, or flower buds.
• many plants will grow and flower with little pruning provided they are given sufficient room to grow. (Don't prune it unless needed!)
  
  Pruning is
• frequently misunderstood and incorrectly practiced.
• Common to wait until a plant has out-grown its intended space before deciding to prune.
• others incorrectly view pruning as an annual spring ritual that must be done --- sometimes when it is not needed.
  
  Pruning should be viewed
• as a regular part of maintenance
• not a remedial correction of neglected problems.

Pruning to maintain plant health includes the elimination of dead, dying, or diseased wood. A dying branch or stub can be an entry point for insects or diseases that could spread to other parts of the plant. When removing diseased wood it is important that the cut be made with a sterile blade into healthy wood. Dip the blades in a disinfectant between each cut or you could spread the disease. (choice of disinfectant – bleach? Listerine? Alcohol? )

Pruning special effect as is the case with espalier, or tree form plants.
Pruning is sometimes undertaken to help a plant recover from transplant shock or from construction damage.
Trees are pruned to eliminate low-growing branches that interfere with traffic or to thin the branches (lacing out) to allow more sunlight to reach plants that are growing under them.

Pruning can enhance the flowering display of some plants. As a plant matures it produces more but often smaller flowers. Pruning reduces the amount of wood and so diverts energy into the production of fewer but larger flowers.
Pruning-off faded flower clusters of crapemyrtle will stimulate the production of new flower buds and a second round of color.
Pruning can be used to produce longer, stronger stems, more colorful stems , or more attractive fruiting.

Response to pruning
A basic understanding of how a plant responds to pruning will help you do a better job. The terminal bud on a shoot produces a hormone (auxin) that inhibits the development of lateral buds along the shoot. When the terminal bud is removed the lateral buds near the cut (6 to 8 inches below) become active and grow. Regrowth on a branch with a 45 to 60 degrees angle will develop farther down the shoot.

When cutting back to a bud, make the cut at a slight angle just above the bud.
NOTE: The angle will allow moisture to flow off the cut. Avoid making the cut at a sharp angle because it will produce a larger wound. When you cut a branch back to the main trunk or to a lateral branch or bud the wound will heal quicker. When you prune leaving a stub the cut is slower to heal. The dead stub is an entrance point for disease and insects.

While pruning is done to reduce the overall size of a plant, it should be remembered that the growth of shoots near the pruning cut are invigorated. Strong shoots should be moderately pruned and weak shoots should be severely pruned. Severely pruning strong shoots will cause vigorous growth.

Correct pruning is the selective removal of branches while maintaining the natural shape of the plant. It is not the same as shearing in which the ends of most, if not all, branches are removed.

A good pruning job should not be conspicuous.
Before starting consider the natural form of the plant; choose branches that do not fit the natural form for removal. Each plant species has an individuality that distinguishes it from other plants.

Shearing increases maintenance; any new growth that occurs will seem out of place and extend beyond the artificial shape that has been created. Pruning to maintain the natural shape is less formal but looks better. New shoot growth tends to blend in and the time between pruning is extended.

Types of pruning
Five basic techniques are used for pruning shrubs:
pinching
heading back
thinning
pruning for renewal
and shearing.
Some plants require more of one method than another, but good pruning is usually a combination of several methods.

Pinching - is the removal of the terminal portion of a succulent, green shoot before it becomes woody and firm. Pinching can greatly reduce the need for more dramatic pruning later on. Whenever (except late summer) you see a shoot becoming excessively long simply pinch or cut the shoot to reduce its length and to promote side branching. Long, vigorous shoots should be cut back into the canopy instead of cut at the outer limits of the existing foliage.

Heading back - involves removing the terminal portion of a woody branch by cutting it back to a healthy bud or branch. Heading back will stimulate shoot growth below the cut thus making the plant more dense. The shape of the plant can be influenced by cutting to inward or outward growing buds. The top bud should be located on the side of the branch that faces the direction new growth is desired. Some plants will have two buds opposite each other on the stem. When such stems are cut, remove one of the buds if you need to control the direction of new growth. If both are allowed to grow, a forked and often weak stem may develop. Repeated heading back with no thinning cuts results in a top heavy plant. Dense top growth reduces sunlight and results in the loss of foliage inside the plant canopy.

Thinning - is the least conspicuous method of pruning and results in a more open plant without stimulating excessive new growth. Considerable growth can be cut without changing the plant’s natural appearance or growth habit. With thinning cuts a branch is cut off at its point of origin from the parent stem, to a lateral side branch, to the “Y” of a branch junction, or at ground level. A good rule-of-thumb is to prune to a lateral that is one-third the diameter of the branch being removed. Thin out the oldest and tallest stems first, allowing vigorous side branch development. This method of pruning is best done with pruning shears, loppers, or a saw --- not hedge shears.

Renewal pruning (rejuvenation) - involves removing the oldest branches of a shrub by pruning them near the ground, leaving only the younger, more vigorous branches which may also be cut back. Small stems (less than pencil size) should be removed.
A variation of renewal pruning involves cutting all branches back to a predetermined height each year. Butterfly bush is often pruned back to woody framework. With time the framework becomes congested and will require some slight thinning.

Shearing - involves cutting the terminal of most shoots with shearing or hedge clippers. This method should not be used on foundation plants but should be restricted to creating formal hedges. Shearing destroys the natural shape of the plant. It causes a thick profusion of growth on the exterior of the plant that excludes light form entering the center of the plant (Figure 15-30). Foliage on the interior of the plant dies. The natural renewal growth from within the plant in prevented.

Time of pruning
Light corrective pruning can be done any time of the year. Occasional, light pruning will reduce the need for severe pruning which might weaken the plant and reduce its natural beauty. When severe pruning becomes necessary it should be undertaken in late winter to early spring.
Woody plants heal pruning cuts by producing rolls of callus that gradually grow over the wound. Internally they compartmentalize or wall-off the damaged tissue from healthy wood. These responses to wounding occur more rapidly just prior to the onset of new growth in the spring and just after maximum leaf expansion in June. The time of pruning will also affect the amount of new growth produced. Plants respond to late winter and early spring pruning by producing vigorous growth to replace the removed wood.
A. When How and Why

1. Pruning normally takes place in the winter months for deciduous trees
2. Sap flow in trees is reduced and the garden calendar is much reduced
3. The 'ideal' window in our world is from the ML King holiday to Valentines Day – it is getting earlier! For citrus, and other non-deciduous trees, this is not true – prune them after they have flowered or fruited regardless the time of year.
4. Trees and some shrubs left to their own devices outgrow the space we've allotted for them, but also grow into a less manageable mass if proper pruning is not done.
   PLEASE NOTE: as you drive around and see government workers and crews pruning trees and shrubs, you can bet money that most of that work is incorrect! Pruning is an art and a science and both have been thrown out the window in a headlong race to hire the lowest bidder.

B. Tools

1. Pruners (a bypass or secatuers not anvil style pruners!), loppers, saw(s), knife, pole saw and pole pruner
   a. pruners are your best weapon – cleanest cut
   b. saws are your second best cut
   c. lopper cuts often need to be cleaned up with a knife
   d. pole pruners and pole saws are difficult to control for correct cuts.
2. Sharp and clean – disinfected too
   a. sharpening demo – hand held stone to the belt sharpener
   b. disinfectants – alcohol, 10% bleach solution, Listerine
3. A ladder for some trees
   a. not your household ladder!
   b. tripod, orchard ladder
4. Dress appropriate to the job at hand
   a. gloves
   b. not a lot of blowy clothes

C. The Principles of Pruning

1. Cut any damaged, diseased or dead wood first.
2. Cut out crossing wood
3. Open the center of the plant to allow light and air flow.
4. Prune to shape, if any more pruning is needed.
5. Cut to a node, paying attention to the apical dominant points, through the bark branch ridge and make the cut as clean as possible.
6. Stop pruning and step back frequently to assess the progress. 'Ask' the tree/shrub what shape it needs to be. DO NOT RUSH. Do not get frozen with anxiety, but don't blithely hack the plant apart.
7. Depend on your pruners mostly with the pocket saw as your second most used tool – use the lopper only as necessary – and use the pole tools the least!

500. Demonstration, indoors and outdoors. Student participation encouraged.

http://www.treesaregood.org/treecare/treecareinfo.aspx