Indole butyric acid (IBA) is one of the most common auxin formulations especially in tissue culture. In
cuttings, it has been found to increase the number of roots, to increase rooting percentage, to increase
both parameters, or to do neither. IBA has had some success in root regeneration in transplanted trees; it
may help redirect resources to the roots by suppressing crown growth.
Naphthylacetic acid (NAA) is also a commonly used auxin and often the active ingredient in commercial
preparations. NAA tends to be toxic to seedling root development, as it inhibits primary root growth and
enhances lateral root growth. This latter activity may account for NAA’s success in regenerating roots of
transplanted and root-pruned trees. Like IBA, NAA apparently
suppresses crown growth, which also may
redirect resources to the roots.
Paclobutrazol (PBZ) is another plant growth regulator that seems to stimulate root growth in
containerized as well as established tree species. Like the auxins, PBZ
reduces crown growth which may
assist with root resources.
Fungicides may increase root growth, but overall this is not beneficial to the plant. Fungicides kill
beneficial mycorrhizal species, and the lack of mycorrhizal colonization means that plants must put more
resources into root growth than they would if mycorrhizae were present. Furthermore, there are
beneficial fungi and bacteria that control pathogenic microbes and roots colonized by beneficial microbes
have been shown to grow more than those without.
Nitrogen supplements can improve root growth, and conversely the absence of nitrogen will depress root
growth. Uptake competition from bacteria, fungi, and other plants can be intense and so nitrogen is often
limiting.
Vitamin B-1 (thiamine) is an important component of tissue culture media, in which isolated plant tissues
can be propagated. Its use for stimulating root growth in whole plants is not supported in the literature
and one study reported that root growth was greater in the control treatment (water) than with thiamine.
Plants in the field manufacture their own source of thiamine and it is therefore unnecessary to add any
additional levels. Many fungi and bacteria associated with plant roots also produce thiamine, so it’s
likely that healthy soils will contain adequate levels of this vitamin without amendment.
Why does the mystique of vitamin B-1 transplant tonics still persist after decades of scientific debunking?
The Bottom Line
• Vitamin B-1, aka thiamine, does not reduce transplant shock or stimulate new root growth on
plants outside the laboratory
• A nitrogen fertilizer is adequate for transplanting landscape plants; avoid use of “transplant
fertilizers” that contain phosphate
• Healthy plants will synthesize their own thiamine supply
• Healthy soils contain beneficial microbes that synthesize thiamine as well
• Difficult-to-transplant species may be aided by application of auxin-containing products in
addition to nitrogen, but read the label and don’t add unnecessary and potentially harmful
chemicals (this includes organics!)
• Adequate soil moisture is crucial for new root growth; be sure to irrigate new transplants
frequently and use mulch to reduce evaporation
For more information, please visit Dr. Chalker-Scott’s web page at
http://www.theinformedgardener.com
