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Deliberate Mechanical Defoliation of Perennials

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A scenario to accompany 'Perennial Plant Response to Defoliation"  and provide an opportunity to apply the concepts learned in that lesson to a real-life problem

Defoliating Perennial Flowers

Patricia Hain
Department of Agronomy and Horticulture at University of Nebraska-Lincoln, USA
Kim Todd
Department of Agronomy and Horticulture at University of Nebraska-Lincoln, USA
2005

This scenario accompanies the online lesson, 'Perennial Plant Response to Defoliation', and is designed to allow you to apply the concepts learned in that lesson to a real-life problem.

Lesson Navigation Tips:

  • To answer questions, select the button next to the correct answer and then select ’check it’ to see if you are correct.
  • To review concepts from the Perennial Plant Response to Defoliation lesson, click on the link below each question.
  • Click once on figures to see enlarged versions.

Marlene Wagner is a new Master Gardener who wants to keep beautiful but large chrysanthemums by her front door from getting so tall that they become leggy and flop over the sidewalk. She has heard that broadleaf perennials can be cut back but she doesn’t know how or when to do this without significantly delaying or losing the flowers or killing the plants.

Asters in Marlene’s garden are flopping over the sidewalk. Image by Patricia Hain, 2005 Marlene would like the asters to be neat and tidy like these. Image by Patricia Hain, 2005

She has also heard people use the words ‘pinching’, and ‘deadheading’, and ‘disbudding’ to describe cutting perennials—all of which sound like methods of inflicting pain. Pinching, deadheading, and disbudding are actually words used to describe slightly different methods of mechanical defoliation. Help Marlene decide whether any of these are appropriate in her situation.

Pinching is removing up to 1/3 of the growth to encourage production of side shoots and reduce height and promote a more dense and bushy habit. This happens because the apical meristem (growing point) also produces hormones that circulate through the plant and suppress branching from lower axillary buds. Removing the apical growing point removes the source of these hormones, and axillary branching is no longer suppressed. This technique delays blooming but the improvements in overall habit and appearance is worth it.

Deadheading is removing spent flowers to reduce seed production and encourage rebloom. A plant’s main goal is to survive and multiply. It does this by producing seed that have the potential to grow into more plants. By removing spent flowers, the plant will continue to try to produce seed by developing more blooms. Also, it will not be putting energy into producing seeds. Rather, it can use this energy to produce more flowers. This is typically used in managing annuals, but also helps keep perennials tidy.

Question: Which of the following is NOT one of the three important things to know in order to determine how the plant will respond after defoliation?
A. Where the growing point is located.
B. Whether the plant is growing in sun or shade.
C. When during the growing season the plant was (will be) defoliated.
D. The amount of green tissue (leaf area) left after defoliation.
To review this concept click on the link: Perennial Plant Response to Defoliation - An Overview: Defoliation of Grasses and Broadleaves.

First, Marlene needs to know WHEN she should cut her asters back to ensure she doesn’t lose the flowers or kill the plant. Asters typically bloom in fall from early August until frost.

Growth stages of a broadleaf perennial. Image by UNL, 2005


 

Question: Would cutting the plant during the flowering stage prevent flowering?
A. Yes
B. No

 
Question: Would cutting back at this time result in a shorter plant and prevent it from flopping over the sidewalk?
A. Yes
B. No
 
Image by UNL, 2005
 
Question: Would cutting the plant during the dormant stage negatively impact flowering?
A. Yes
B. No

 
Question: Would cutting back at this time result in a shorter plant and prevent it from flopping over the sidewalk?
A. Yes
B. No
 
Image by UNL, 2005
 
Question: Based on the answers to the previous questions, during which of these growth stages should Marlene cut her asters back to accomplish her goals of a shorter plant that won’t flop over the sidewalk and will still flower at about the normal time?
A. vegetative
B. bud
C. flowering
D. dormant

Now that Marlene knows WHEN to cut her asters back, she needs to know WHERE to make the cut so she can accomplish her goal of keeping the asters shorter without losing the blooms.

Question: Where is/are the growing point(s) for most broadleaf perennials located?
A. There is one growing point at the apex (tip) of the shoot.
B. There is one growing point at the base of the shoot.
C. There are many growing points along the shoot.
D. There is one main growing point at the apex and many others along the shoot.
Question: Why is the amount of green tissue (leaf area) remaining important?
A. The more leaf area that is removed, the more the plant must regrow.
B. Flowering could be significantly delayed.
C. More leaf area means more potential for photosynthesis.
D. All of the above
To review these concepts click on the link: Perennial Plant Response to Defoliation - An Overview: Defoliation of Grasses and Broadleaves.
Question: What would happen (how would new growth occur) if Marlene cuts the plants back below all growing points as indicated in the picture?
A. New buds and leaves would develop along the internode of the remaining part of the shoots.
B. Growth of the current shoots would cease and new growth would have to occur from dormant buds at the crown.
C. The plant would resume growing at the cut on the top of the shoot, developing new buds and leaves.
D. The plant would die because all of the growing points were removed and no new growth could occur.

 
Question: Would cutting back the plant this amount result in a shorter plant and prevent it from flopping over the sidewalk?
A. Yes
B. No

 
Question: Would cutting the plant this amount negatively impact flowering?
A. Yes
B. No
 
Image by UNL, 2005
 
Question: What would happen (how would new growth occur) if Marlene cuts the plants back “here” (about 1/3 of the mass of the plant or pinching back)?
A. New branches would grow from remaining axillary buds.
B. The plant would resume growing at the cut on the top of the shoot, developing new buds and leaves.
C. Growth of the current shoots would cease and new growth would have to occur from dormant buds at the crown. (which would take a long time?)
D. The plant would die because all of the growing points were removed and no new growth could occur.

 
Question: Would cutting back the plant this amount result in a shorter plant and prevent it from flopping over the sidewalk?
A. Yes
B. No

 
Question: Would cutting the plant this amount negatively impact flowering?
A. Yes
B. No
 
Image by UNL, 2005
 
Question: What would happen (how would new growth occur) if Marlene deadheads the plants?
A. New branches would grow from remaining axillary buds.
B. Growth of the current shoots would cease and new growth would have to occur from dormant buds at the crown.
C. New blooms would develop at the remaining axillary buds.
D. The plant would die because all of the growing points were removed and no new growth could occur.

 
Question: Would dead headind the plant result in a shorter plant and prevent it from flopping over the sidewalk?
A. Yes
B. No

 
Question: Would cutting the plant this amount and in this way negatively impact flowering?
A. Yes
B. No

 
Question: Based on the previous questions, at which height should Marlene cut her asters?
A. Below all growing points.
B. Cut off the top 1/3 of the plant.
C. Deadhead the flowers.
D. It doesn’t matter.

 
Image by UNL, 2005
 
To review these concepts view the: Perennial Plant Response to Defoliation Animation.
Question: If a plant was cut back below all the growing points during the flowering stage, what would happen to the carbohydrate reserves for the winter?
A. increase
B. decrease
C. stay the same
To review this concept view the: Perennial Plant Response to Defoliation Animation.
Question: How could this affect the health of the plant next spring?
A. hurt it
B. help it
C. no affect
To review this concept click on the link: Perennial Plant Response to Defoliation - An Overview: Carbohydrates and Defoliation.

Development of this lesson was supported in part by the Cooperative State
Research, Education, & Extension Service, U.S. Dept of Agriculture under Agreement Number PX2003-06237 administered by Cornell University, Virginia Tech and the American Distance Education Consortium (ADEC) and in part by the New Mexico and Nebraska Agricultural Experiment Stations. Any opinions,findings, conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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