Weeds, Weedkiller and Bonsai (Why Weedkiller and Bonsai Don't Mix)

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So Why Shouldn't Glyphosate or RoundUp Be Used On A Weed Found In A Bonsai Pot?

Glyphosate/Round Up should become inert and/or heavily diluted once it comes into contact with the soil in the bonsai pot. However it can become 'locked up' in the soil and remain active for a period of time. As there is no warning whether the weedkiller has or hasn't broken down successfully in the soil, it is very possible for a bonsai in the confines of a small pot to be affected by any locked up weedkiller residue without the enthusiast realising; with disastrous consequences.

According to IPM of Alaska "Glyphosate is moderately persistent in soil, with an estimated average half-life of 47 days. Reported field half-lives range from 1 to 174 days. It is strongly adsorbed by most soils, even those with lower organic and clay content. Thus, even though it is highly soluble in water, field and laboratory studies show it does not leach appreciably, and has low potential for runoff."

(Half-life is the decay rate defined as the length of time for 1/2 of the initially measured residue to degrade)

Such a long half-life (possibly nearly 6 months) in such a small volume of soil makes the possibility of exposure to Glyphosate by the roots of a bonsai even greater.

Sinclairs 'Diseases of Trees and Shrubs' warns that " in soilless media or pure sand crops (read bonsai) injury from root uptake has been observed...

Symptoms of herbicide damage: growth suppression, distorted leaves and shoots, chlorosis or bleaching, browning, leaf drop, dieback.... Very sensitive plants include: Siberian elm, willow, hackberry, grapevine, boxelder and redbud.

Some herbicides may trigger damage by secondary factors. For example, some herbicides reduce cold hardiness..."

Most worryingly, damage to bonsai by the use of weedkillers can be greatly delayed "......Injury to plants receiving small amounts of drift may be expressed one to two years after the occurrence".

Undoubtedly, glyphosate and Round UP can and have been successfully used to kill weeds in bonsai pots for many years but there is a real risk of damage or death via soil contamination. The possible delay in reaction makes pinpointing the cause to the use of a weedkiller difficult.

In Conclusion

I would strongly suggest that weeds are regularly removed by hand as they emerge.

Established and/or deep rooted weeds should be cut back as hard as possible without trying to remove the roots. Such weeds can be easily and successfully removed at the next repotting of the bonsai.

Avoid the use of weedkillers; the risk of killing your bonsai using glyphosate or Round UP may be relatively small but it does exist.

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