Garden Myths Busted: Potassium / Magnesium+ Cold, Native Plants, Cedar Wood Chips
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Garden Myths Busted: Potassium / Magnesium+ Cold, Native Plants, Cedar Wood Chips

By Paul Rodman (paulgrow)November 14, 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

The Myth:

Adding potassium or magnesium to your landscape plants will increase their cold hardiness


The Facts

Several articles have been written recently for gardeners who like to "push the zone" envelope. With winter weather on the way, this information is generally useful for protecting marginally cold-hardy plants, The addition of wood ashes (for potassium) and Epsom salts (for magnesium) was recommended to promote cold hardiness of plants grown outside their normal range.

In particular, potassium is mentioned numerous times as "promoting cold hardiness, disease resistance, and general durability."  Some horticultural consultants state "what the potassium does is strengthen the cell walls of the plant and will displace the amount of water in the cells making it harder for them to burst during freezing temperatures."

They also recommend that by adding magnesium for increased cold hardiness is less common on the web, and the rationale behind its use is not clear. Where did these recommendations originate?

We first need to dispel the notion that freezing temperatures cause plant cells to burst. This is a common misconception, but in fact rarely happens in nature. What generally happens is that water freezes in the spaces between the cell walls, and this ice formation draws liquid water from the living cells. The living ells are actually stressed and sometimes killed by the dehydration imposed by water moving into these paces.

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  • Before adding any mineral supplement to your landscape, have a soil test done first to determineif deficiencies exist,
  • Addition of chemicals (organic or inorganic) to a landscape where no mineral deficiency exists is a waste of money, time, and resources, and is environmentally irresponsible.
  • There is no conclusive evidence that addition of either potassium or magnesium will increase the hardiness of native landscape species
  • There is no evidence whatsoever that addition of either potassium or magnesium will increase the hardiness of non-native, marginally hardy landscapeplant.
  • To grow marginally hardy species, take advantage of microclimates to maximize their chances for survival.
  • The best strategy for overwintering marginally hardy species is to insulate them and the surrounding soil.

The Myth


Always choose native plants for environmentally sustainable landscaping

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The Facts
One of the most populr trends in gardening circles is to use more native plants as our natural ecosystems shrink.
There is a large misconception that native plants are supposed to be be resistant to local pathogens and parasites.

There are some urban areas where many native plants just do not survive (or do so only with substantial maintenance). Such areas can include parking strips, traffic circles, and parking lots: in short, areas with imited soil area and lots of environmental stress. Consider the realities of these landscapes:

Dissimilar layers of topsoils and subsoils with poor drainage and aeration

  • Significant compaction and other physical disturbances as a result of animal, pedestrian, andvehicular traffic
  • Alkaline pH due to leaching of lime from concrete
  • Inadequate or improper fertilizer application
  • Lack of mulch or other soil protection
  • Lack of adequate water in summer months
  • Increased heat load from asphalt reflectance
  • Air pollution
Many of the trees and shrubs native to your region evolved in thin, acidic soils with adequate moisture to maintain soil and plant water status. When these species are installed in urban landscapes with significantly different soil and water characteristics they are challenged by a new set of environmental circumstances.Native, forest plants are excellent choices for unrestricted sites with acidic, well drainedsoils.
  • For sites with limited, alkaline, and/or poorly drained soils, choose species adapted toenvironments with similar soils. Consider especially those species that tolerate clay soils.
  • For sites exposed to increased heat choose species adapted to hot, dry climates that can also tolerate cool, wet winters.
  • Instead of installing large trees into limited sites, considered smaller trees or shrubs, Be sure to protect soils with mulch, especially where foot traffic causes compaction.
  • Site considerations should always dictate plant selection.

The Myth

Wood chips made from cedars will kill landscape plants

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The Facts

Many gardeners are reluctant to use cedar wood chips because they will kill plants.When one plant emits toxics that kill other plants this process is called allelopathy. One of the best known allelopathhic plants is the Black Walnut tree. It is believed that plants do this to eliminate competition for water and nutrients.

There is virtually no documented evidence for allelopathic activity in cedars.

  • It is unlikely that wood chip mulches containing cedar will have negative effects on established landscape plants.
  • The allelopathic activities attributed to mulches made from cedar and other species may actually be due to other factors such as nutrient and light limitations.
  • Seeds and seedlings, whether weeds or desirable species, are more sensitive to mulch suppression as they do not have established root systems.

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
Enjoy your articles leemiller38 2 8 Nov 21, 2012 5:08 PM
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