Stress Is Good For Plants

Fig. 1 Petunias finished  in cooler temperatures in an unheated high tunnel (left) were more compact than plants finished in a heated greenhouse (right).

Do your greenhouse plants live a life of luxury? Soaking up the sun, bathed in nourishing nutrients, in a temperature-controlled climate — your greenhouse is like Club Med for plants. But then reality hits as the plant is shipped to the retail environment and then brought into the “real world,” when the consumer brings it into his or her home or landscape.

A plant that has been coddled in the greenhouse may not perform as well once taken to a more stressful environment. What’s a grower to do? One strategy is to purposely stress plants to make them more compact or enhance their ability to thrive once they hit a more stressful environment.

Plant Stress, Defined
Stress, as far as plants are concerned, can be generally split into two different groups: biotic and abiotic. Biotic stress is one caused by another living organism, such as disease or insect infestations. It is important to avoid biotic stresses as they can quickly spread from plant to plant and render a crop unmarketable.

Abiotic stress is one caused by non-living factors, including high- or low-temperature stress, drought stress, over- or under-fertilization, high salts and high or low light. While exposing a plant to abiotic stress may reduce productivity, when applied appropriately and in moderation, it is one way to keep a crop more compact, reduce its water requirements and improve its ability to withstand further stresses once it leaves your operation.

Let’s say you are growing a crop in flats or pot-to-pot and it is a crop you do not want to re-space. As stems and leaves grow, the plant canopy fills out the allotted space. As a plant’s leaves come into contact with its neighbors, the plant will undergo a “shade avoidance response.” This is an internal trigger causing stems to stretch more in attempt to reach the sun and avoid shade from its neighbors.
Plant growth regulators are one tool to control excessive growth, but another tool for keeping growth in check is exposure to a moderate temperature or water stress.

Temperature Stress Controls Height And
Delays Flowering

Most plants have an optimum temperature at which their development rate is the quickest. At the optimum temperature, unfolding of new leaves and progress toward flowering is the quickest. Relatively warm temperatures also promote elongation of stems and leaves.

Growing a crop cooler reduces its development rate, thereby reducing the number of nodes on a stem and limiting the stem elongation of each node. This leads to an overall reduction in stem length. A trade-off with low temperatures is that the plant will take longer to reach flowering, so using low temperatures to keep height in check may be a good strategy for flower crops once they have nearly reached flowering. One technique for bedding plant crops is to “tone” them by exposing them to lower-than-optimal night temperatures beginning at visible bud. For cold tolerant crops such as ageratum, dianthus, pansy, petunia and snapdragon night temperatures of 50°F to 55°F may be used. For cold sensitive crops such as begonia, celosia, coleus, impatiens and vinca, try night temperatures of 58°F to 62°F.

Another approach to using lower temperatures that we have collectively been researching at Cornell University and Purdue University, is using unheated high tunnels to finish bedding plant crops. We transplanted ‘Dreams Midnight’ petunias into 4-inch pots on April 1 and moved plants to either greenhouses heated to 65°F or high tunnels. Plants were grown until they were in flower and considered marketable, which was about mid-May.

Compared to the greenhouse, plants in the unheated high tunnel had an average temperature about 5 degrees cooler, but with much greater extremes varying from about 27°F on cold nights to more than 100°F on sunny days. Flowering of the high tunnel plants was delayed by about one week, compared to their greenhouse counterparts, but they were much more compact, at 4½ inches tall compared to 9 inches tall (Figure 1).

Drought Stress Manages Size And Improves Hardiness
Mild to moderate drought is another way to stress plants to help manage their size and improve their hardiness once purchased by the consumer. A plant grown with luxuriant water will grow bigger with softer stem and leaf tissue than one with mild drought stress.

Leaf wilting is a common symptom of mild drought stress. Stressing a plant to the point of wilting can be dangerous to a crop. If wilting is allowed to continue, the plant may become so dehydrated that it cannot recover. This is called the permanent wilting point (Figure 2). The intensity of water stress required to reach this point varies by crop. Severe water stress also causes the effective salt concentration on a root-zone to rise to dangerous levels, which can damage roots.

A more subtle way to apply water stress is to use chronic or low-level, but long-lasting, water stress. One strategy, long-used by growers, is to allow the root-zone to dry until a plant nearly wilts before watering it again. Instead of fully watering the container, a lower amount of water is used.
A more formal way to implement this is called Regulated Deficit Irrigation (RDI). In RDI, a grower purposely limits the amount of water given to the plant to some percentage of its actual needs. RDI has been shown to reduce the internode growth of some nursery crops, reducing the need for mid-season pruning. RDI can improve commercial crop quality, save water and yield plants better adapted to dry environments. A disadvantage of RDI is that it can limit ornamental crop marketability by reducing leaf and flower area.

At Cornell we used RDI to grow ‘Prestige Red’ and ‘Peterstar Red’ poinsettias. A control group of plants received 100 percent of water needs. Representative control plants were weighed to determine how much water they used each day and RDI treatment plants received either 60 percent or 80 percent of control plant needs. We implemented RDI using drip irrigation.

For example, if we estimated that control plants needed 5 minutes of water during an irrigation event, 80 percent plants received 4 minutes and 60 percent plants received 3 minutes of water. RDI was effective at reducing plant height; plants were 1 to 2 inches shorter than their control counterparts (Figure 3).

The overall size of the plant, including bract surface area, was also reduced by RDI. Bracts of 80 percent RDI plants were about 25 percent smaller and bracts of 60 percent RDI plants were about 50 percent smaller than control plants. However, a major benefit was that RDI plants used less water and therefore took longer to wilt in the postharvest environment.

Treatment plants were moved to a simulated retail environment. Plants were well-watered once and then we recorded the number of days to wilt. RDI plants lasted three to four days longer before wilting.

Our results suggest that a moderate RDI can restrain plant growth during production and result in a longer-lasting plant for the consumer.  GG

Leave a Reply

More From Plant Culture...
PP&L CAST 2015 intros

April 22, 2015

6 Breeding Companies Serve Up New Varieties At Pacific Plug & Liner

Pacific Plug & Liner’s theme this year, Labyrinth, a conservatory of the world’s most captivating plants, was perfectly topped off (pun intended) with fascinators for the women and newsboy caps for the men. The PP&L team dressed their part to act out the gothic “conservatory of the world’s most captivating plants.” Truly, the displays looked like they practically popped out of a catalog, and the costumes were a nice touch. Retailers take heed, the fully merchandised displays at Pacific Plug & Liner are worthy of emulating. We’ll let the pictures tell the story of all the fabulous variety introductions presented at  Pacific Plug & Liner’s 2015 California Spring Trials, where Cultivaris, Cohen Nurseries, Histil Nurseries, Jaldety Nurseries, Southern Living/Sunset Collection and Pacific Plug & Liner all highlighted their 2016 introductions.  

Read More
Speedling 2015 CAST intros

April 22, 2015

Speedling Inc. Presents New Varieties From ABZ Seeds, Hem Genetics, Thompson & Morgan, Vista Farms & PSI

You name it, we saw it at Speedling's California Spring Trials location in San Juan Bautista, where five companies showed off their new introductions for 2016.

Read More
PittMoss on Shark Tank

April 22, 2015

PittMoss Wins On Shark Tank

Mont Handley, president and CEO of PittMoss, appeared on ABC’s Shark Tank on April 17 to try to get the “sharks” to invest in his peat moss alternative. Three investors from the TV show contributed $600,000 to PittMoss for a 35 percent stake in the company. Check out this clip from ABC’s website in which Mark Cuban, Kevin O’Leary and Robert Herjavec discuss getting on board with the product. PittMoss is an alternative to sphagnum peat moss, made up of a mix of proprietary additives and recycled paper rescued from landfill space. Handley founded the Pittsburgh-based company in 1994. What started as a small experiment grew into a full-fledged business with the help of funding provided by an EPA SBIR grant and Pittsburgh’s Idea Foundry. Today, PittMoss is available to commercial greenhouses and nurseries from Michigan to Maine to North Carolina, with plans to grow. To learn more, visit PittMoss’ website, or check it […]

Read More
Latest Stories
Bee on a Sedum

March 17, 2015

4 Key Pollinator Research Projects To Be Funded By Hort…

The Horticultural Research Institute will grant $125,000 in financial support for four key projects as part of the Horticultural Industry Bee & Pollinator Stewardship Initiative. The Initiative has three primary goals. First, to convene a task force to develop a bee and pollinator stewardship program, including creation of best management practices for plant production. Second, to identify and fund research that will help answer key science questions and fill gaps needed to design and refine the stewardship program. Third, to seek to positively position the horticultural community and its customers by collaborating with other compatible groups interested in augmenting pollinator habitat and protection.

Read More

March 11, 2015

Pollinator Initiative Promotes Bee-Friendly Talking Poi…

AmericanHort and the Society of American Florists are working tirelessly with the ornamental industry's Pollinator Stewardship Initiative on a number of new projects.

Read More

February 11, 2015

Infusion Technology Boosts Seed Performance, Study Sugg…

Seven-year-old wheat seed germination can increase by as much as 83 percent, according to a Vital Force Technology Study that looks at the effects of energy infusion technology on plant vitality.

Read More

February 3, 2015

American Floral Endowment Accepting Research Pre-Propos…

If you are pursuing a floriculture research project, now is the time to apply for funding through the American Floral Endowment. Research pre-proposal applications for 2015-2016 funding are due to AFE by June 1, 2015.

Read More

January 27, 2015

Marijuana’s Trajectory And Ascent To Horticultural Cr…

Marijuana growing is poised for change as growers and researchers focus on improving production practices.

Read More

December 9, 2014

Greenhouse Production: Two Years Of Basics & Beyond…

Greenhouse Grower's Basics & Beyond articles cover some of the latest news and research going on in greenhouse production. Here are article links for the last two years.

Read More
GrowIt! App Wins Gold At Design100 2014 US Mobile & App Design Awards

November 24, 2014

GrowIt! App Wins Gold At Design100 2014 US Mobile &…

The social garden app GrowIt! takes the Gold Winner award at the design100 2014 Mobile & App Design Awards.

Read More

November 10, 2014

The Perennial Farm Joins HGTV HOME Plant Collection

The Perennial Farm joins the HGTV HOME Plant Collection growers' network for 2015.

Read More
AmericanHort

November 4, 2014

AmericanHort Publishes Revised American Standard For Nu…

AmericanHort announces the revised American Standard for Nursery Stock (ANSI Z60.1) is now available for industry use. The Standard reflects the consensus of the industry regarding how nursery stock — living plants other than annuals — should be specified and sold within the trade.

Read More

September 26, 2014

Master The Art Of Watering

Watering is elemental to healthy plants, but one of the hardest concepts for new employees to master in the greenhouse. Recommend these tips to start them off right.

Read More

September 16, 2014

Ball FloraPlant’s Las Limas Facility Provides Gro…

Ball FloraPlant’s Las Limas farm in Esteli, Nicaragua, is one year away from full production, but sales and quality from the two-year-old facility are right on track.

Read More
Erysimum 'Cheers' from Darwin Perennials

September 15, 2014

Darwin Perennials Takes Production Offshore In Bogota, …

With its recent purchase of a farm in Colombia, Darwin Perennials is ready to amp up supply of its perennial genetics, to provide growers with tried-and-true varieties and comprehensive production specifications.

Read More

July 23, 2014

Plan Now To Prevent Bract Edge Burn On Poinsettias

Reduce fertilizer and water, and allow your poinsettias to develop slowly during the final four weeks of production to avoid bract edge burn.

Read More

July 11, 2014

Growing Your Crops Above Their Base Temperature

Lowering temperature set points in the greenhouse may help you combat rising heating costs.

Read More

May 1, 2014

Growers Report Nutritional Problems On Geraniums

In recent weeks, several growers have contacted Michigan State University Specialists about leaf discoloration on geraniums, especially the purpling of lower leaves.

Read More

April 22, 2014

How Two Postharvest Care Products Worked On Potted Plan…

What your potted plants look like at retail translates to sales or fails. North Carolina State University researchers report on how two postharvest care products performed.

Read More
Dianthus 'Passion' from Emerald Coast Growers

March 27, 2014

Growing Dianthus Successfully

Here's some advice on transplanting and producing this classic perennial favorite.

Read More
Aquilegia canadensis

March 10, 2014

Tips For Producing Aquilegia

Advice on planting, temperature, vernalization, lighting and more on columbine from Emerald Coast Growers' head grower Josiah Raymer.

Read More