Debunking Horticultural Myths

green lacewing, predator of insect pests but not a panacea

green lacewing, predator of insect pests but not a panacea

In the February 2006 edition of  “Landscape Notes,” the Ventura County Cooperative Extension newsletter, Dr. James Downer, Ventura Country farm adviser, writes a fascinating article titled  “Snake Oil, Horticultural Myths, Horticultural Urban Legends and Persuaders in Our Industry.” Downer’s thesis, backed up by research, is that many garden products, both old and new, as well as certain long-accepted horticultural practices, are either ineffective or counterproductive in the garden.
The use of these products and practices is based on spurious science, misleading advertising hype or pure myth. Walk into almost any nursery or garden center and ask, “How can I reduce transplant shock?” and, nine out of 10 times, you will be pointed in the direction of Vitamin B-1, which is bottled by a variety of manufacturers.
For more than 50 years, vitamin B-1, also called thiamine, has been touted as the preferred potion for recently planted, transplanted, stressed or struggling plants. It all started in the 1930s when a Cal Tech scientist used B-1 to revive root tips whose development had been arrested when growing in tissue culture in a laboratory. Based on this sliver of evidence, Better Homes and Gardens magazine praised this practice in 1939, claiming that you could grow giant roses, daffodils and snapdragons through its application. The rest, as they say, is history. All this despite the fact that a few years later, the same Cal Tech scientist who first used B-1 to positive effect wrote:   “It is now certain that additions of vitamin B-1 to intact growing plants have no significant or useful place in horticultural or agricultural practice.”
Interestingly, many garden products that contain B-1 and that are sold as growth stimulants do perform as advertised. However, this is not because of the B-1. Growth stimulants that do work owe their success to the presence of plant growth hormone, identifiable by the acronym IAA or IBA on the label.
During the last decade or so, many wheelbarrow loads of advertising dollars have been spent lauding the application of mycorrhizal fungi in the garden, especially when planting trees. Everyone agrees that many plant species cannot grow without these beneficial microorganisms – which make certain mineral elements, especially phosphorus, available to roots in the soil. However, this does not mean that such organisms are not already present in the soil or that their application makes a significant impact on growth.
Biological insect control is critical to our ability to raise plants and crops of all kinds. In nature, beneficial predatory insects such as ladybugs, lacewings and parasitoid wasps keep insect pests under control. This has led to a huge market in selling such insects to gardeners. Yet, according to Downer, “biological control is an elusive thing that we … consistently fail to re-create when and where we want it to happen.”
Five years ago, the landscape maintenance contract for a local university specified that a certain quantity of lacewings should be released at regular intervals to achieve pest control in the landscape. Last year, when the contract was rebid, there was no longer any such specification for one reason: The lacewings did not deter pests. As Downer indicates, biological control with predatory insects is an inexact science.
Placement of gravel at the bottom of planting holes has long been advocated as a prudent horticultural practice in order to improve drainage. Yet, this makes no sense. If soil at a 2- or 3-foot depth drains poorly, what difference could a few inches of gravel possibly make?
It is also commonly thought that pruning stimulates growth. But when latent buds are forced open beneath pruned shoots, there is actually less trunk growth on a pruned than on an unpruned tree. For this reason, it is ill advised to prune a tree after planting it, even though this procedure is recommended, as if it could compensate for roots lost during planting.
Lastly, it has always been said that woody mulches “rob nitrogen from the soil.” In fact, the opposite is true, since decomposing mulch eventually releases nitrogen that can be absorbed by roots.

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