Garden Myths Busted: Artiifical Light, Milk Spray, The Bottom of the Pot
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Garden Myths Busted: Artiifical Light, Milk Spray, The Bottom of the Pot

By Paul Rodman (paulgrow)December 26, 2012
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Two horticultural professionals took some of the most popular garden myths into the university laboratory to prove or disprove the accuracy of these myths. Dr. Linda Chalker-Scott from Washington State University and Dr. Jeff Gilliam from the University of Minnesota tested these myths under controlled conditions to determine if they really work.

Gardening picture

Myth:

Prolonged exposure of plant material to artificial light especially high-intensity lights can hinder a plants growth.


Fact:

Since the beginning of time plants have evolved by adapting to their environment. The changing temperatures as the seasons progress and the length of daylight that they are exposed to are just a couple of the factors that plants adjust to.  We all know that the first day of summer is the longest period of daylight that we encounter. On the other hand the winter solstice is the shortest period of light during the year. Man has taken it onto himself to provide artificial lighting for a number of purposes from home security to advertising signs. Recently so called improvements in lighting have resulted in very high intensity lighting. I'm sure that we have all encountered the high intensity headlights that have been installed on automobiles during the past decade.


Does exposing plants to this artificial high intensity lighting affect them? It certainly does.


When urban trees, especially street trees, are exposed to extended light periods, those leaves and buds nearest the light source perceive an endless summer and keep on growing. This phenomenon is difficult to see initially. In the autumn it is quite distinctive. Affected leaves retain their green color while those leaves under natural conditions have already started to senesce and change colors.  When the first autumn frosts arrive, these green leaves die and the trees lose the resources that normally are scavenged during senescence. Recent research has demonstrated that high-intensity light sources, such as high-pressure sodium lamps, have the greatest impact on delaying leaf senescence and subsequent dormancy of landscape trees.

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Artificially prolonged light periods can interrupt flowering cycles and delay winter dormancy.

When planting trees and shrubs consider the following:

  • Marginally hardy plant materials should never be exposed to interrupted dark periods.
  • Look at the location of high-intensity light sources (street lights, security lights, etc.) before
  • The effects of high-intensity lights can be partially moderated by installing deflectors on the lights.

Myth:

Spraying milk on plants will help to prevent foliar diseases.

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Fact:

This myth began as a result of a Brazilian study published in 1999 focusing on powdery mildew control on zucchini. This new alternative to conventional fungicides has been combined with reports of successful powdery mildew control on a variety of plants, including roses.

The treatment is also touted as preventing leaf black spot, thus giving hope to rose growers everywhere as a safe, effective method of growing disease-free plants, including roses.

Milk has been used for many decades; for instance, it has been used with varying effectiveness as a spreader or sticker in pesticide applications. Perhaps the best-documented use of milk has been in reducing the transmission of leaf viruses, especially tobacco mosaic and other mosaic viruses.

There are a few potential drawbacks to using milk as a foliar spray:
  • Milk-fat can produce unpleasant odors as it breaks down.
  • Fungal organisms can  colonize on leaves and break down 
  • Dried skim milk has been reported to induce black rot, soft rot, and Alternaria leaf spot on treated and Alternaria leaf spot on treated cruciferous crops.

Is it worth trying milk as a treatment for viruses, powdery mildew, or any other disease? Absolutely!

There is evidence that milk treatments can be effective in the protection of some crops, and organic growers especially might benefit from this method. 

What it means to you:

  • There is no evidence that milk sprays are effective in controlling black spot on roses or any other ornamental plant species.
  • Milk sprayed onto leaves may act as a nutrient source for benign microorganisms, decreasing the leaf area available for powdery mildew to infect.
  • Leaves coated with a milk spray may be less vulnerable to aphid attack, thereby reducing the transmission of aphid-borne viruses.
  • Milk sprays can encourage the growth of other microorganisms, whose presence may be aesthetically unappealing.
  • Milk sprays may be a viable alternative to conventional pesticides, especially for organic farmers.

If you would like to give it a try here's a recipe.

1. Mix 1 cup of milk with 9 cups of water.
2. Put the solution in a sprayer and spray the entire plant including the undersides of the leaves.
3. Do this twice a week and your powdery mildew problem should disappear.

Milk works in two ways: It has a germicidal effect--it kills the fungal spores and it also appears to stimulate plants in such a way that they become more resistant to the disease. In recent university tests, the milk and water spray was found to be more effective than the two most popular synthetic fungicides on the market today.

Myth:

Placing gravel or pot shards in the bottom of a container improves drainage

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Fact

Scientists demonstrated long ago that water does not move easily between fine- and coarse-textured soils and materials.

Water won't move from potting soil into coarser-textured gravel or a shard until the soil is saturated. Adding gravel can create the water-logged state it's intended to prevent.

Personally I like to use a coffee filter; this prevents the potting medium from leaking from the drainage hole while still allowing adequate drainage.

In closing, I expect some of you might disagree with some of these findings. However remember that this research was done in reputable university labs under controlled conditions. Each of us will draw our own conclusions.


  About Paul Rodman  
Paul RodmanPaul Rodman has been gardening for over 45 years. He is an Advanced Master Gardener, and American Rose Society Consulting Rosarian. He is President Emertius of the Western Wayne County Master Gardener Association in Wayne County, Michigan. He currently serves as the greenhouse chairman of this group. Rodman has amassed over 5500 volunteer hours in the Master Gardener program. Rodman is the garden columnist for The News Herald newspaper, in Southgate, Michigan. He has also written for the Organic Gardening.com web site. He is a certified Master Canner and has taught classes on Home Food Preserving for 7 years. He has lectured on various gardening topics throughout southeastern Michigan. His favorite pastime is teaching children about gardening. For the past several years he has conducted classes for second grade students teaching them about subjects ranging from vermi-composting to propagation.

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Discussion about this article:
SubjectTopic StarterRepliesViewsLast Post
Garden Myths weedsfree 11 102 Jan 24, 2013 4:21 PM
coffee filters joandud 1 19 Jan 3, 2013 8:27 PM
Gravel Baja_Costero 5 95 Jan 1, 2013 9:28 PM
I grow veggies under lights 24hrs a day ElementalDomain 0 17 Jan 1, 2013 6:55 PM
Milk works water_sun_grow 0 20 Dec 31, 2012 10:21 AM
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