Ivy Eradication: A Comedy of Errors

When the concert meadow in San Francisco’s Stern Grove was renovated in 2005, at a cost of $15 million, we were surprised that ivy was planted as the ground cover because ivy grows rampant in Stern Grove, shrouding many of the trees.  But, hey!  Who are we to question the choices of horticultural professionals? 

Ivy planted in Stern Grove, 2005

Now ivy is being sprayed with herbicide–presumably with the intention of killing it–by San Francisco’s Natural Areas Program in other parks in San Francisco, so one wonders if the staff who plant it are aware of the future of the ivy they plant.  Seems like another case of man creating problems which he then must solve.  Perhaps full employment is the objective, rather than the creation of a beautiful garden.  But we digress.

Ivy climbing trees in Stern Grove

Combining pesticides

Many members of the public are of the opinion that all pesticide (herbicides, insecticides, etc.) applications are inappropriate in a park that has been designated as a “natural area.”  Last year, the public complained about the spraying of Garlon in the natural areas by the Natural Areas Program because it is classified by the city’s Integrated Pest Management (IPM) policy as “Most Hazardous.”  Consequently, the Natural Areas Program significantly reduced its use of Garlon in 2011. 

For the most part they have substituted a mixture of glyphosate and imazapyr for Garlon. Is this an improvement?  Maybe not.  Although glyphosate and imazapyr have a lower hazard rating of “More Hazardous,” the Natural Areas Program increased their pesticide applications in 2011 at least 20% compared to 2010.  But more importantly, little is known about the toxicity of imazapyr and nothing is known about the toxicity of combining glyphosate and imazapyr.(1)  Imazapyr was approved for use in California in 2005, so only the minimal tests required by law have been done on it.

The manufacturer’s labels for these herbicides suggest that combining them is not an approved use.  The label for Aquamaster (glyphosate) does not include imazapyr on the list of pesticides with which it can be safely combined.  And the Polaris (imazapyr) label says it should not be combined with another pesticide unless it is expressly recommended by the manufacturer of that pesticide.

The “Aquatic Pesticide Application Plan for the San Francisco Estuary Invasive Spartina Project” is cited by San Francisco’s IPM program as the evaluation upon which it based its decision to add imazapyr to the list of pesticides approved for use in San Francisco in 2010.   The evaluation explained why imazapyr is being combined with glyphosate by the non-native Spartina eradication project. 

Imazapyr is apparently slow acting.  It can take some months before it kills the plant on which it is sprayed.  Glyphosate, on the other hand, is fast acting.  The plant on which it is sprayed begins to yellow and die within a few weeks.  Glyphosate is therefore used by the Spartina eradication project to provide quicker feedback to those spraying the herbicide.  They know within a few weeks if they have sprayed in the right place.  They don’t have to wait for the next season to spray again if necessary. 

Pesticide Application Notice, Glen Canyon Park, December 2011

 However, glyphosate should be applied to perennial broadleaf plants during their reproductive stage of growth, when they are budding in the late spring and summer, according to the manufacturer.  In Glen Canyon Park, a mixture of glyphosate and imazapyr was sprayed on ivy in December 2011, clearly not the recommended time period for spraying.  A month later, there is no indication that the ivy was damaged by this spraying.  This suggests that there was no point in combining glyphosate and imazapyr in this application.  The public was exposed to the unnecessary risk of combining these herbicides, with no potential benefit of taking that risk.


Pesticides accumulate and persist in the soil

Was it appropriate for the city’s IPM program to use the evaluation of imazapyr for the Spartina project as the basis of their decision to approve its use by the Natural Areas Program?  We don’t think so.  The circumstances of the Spartina project are substantially different from those of its use by the Natural Areas Program.

Imazapyr is used to eradicate non-native Spartina in a tidal estuary.  For that reason the evaluation of its use assured the public that this herbicide would not accumulate in the environment because it would be flushed away from the ground by the tide twice each day. 

The evaluation also said that when imazapyr was used in a pond or stable water source, it persisted in the ground for a longer period of time.  In fact, that’s exactly how imazapyr is being used by the Natural Areas Program.  It has been used at Lake Merced and at Pine Lake, both stable water sources.  It is also being used in Glen Canyon Park, which is a watershed. 

We don’t assume that imazapyr is being used safely to eradicate Spartina.  However, even if it is, it does NOT follow that it is safe for use in watersheds that are not tidal, such as those being sprayed by the Natural Areas Program. 

Collateral damage of pesticides

Glyphosate is a non-selective herbicide.  That is, it kills any plant it is sprayed on at the right stage of its growth.  But imazapyr is far more insidious as a killer of plants because it is known to travel from the roots of the plant that has been sprayed to the roots of other plants.  For that reason, the manufacturer cautions the user NOT to spray near the roots of any plant you don’t want to kill.  For example, the manufacturer says explicitly that imazapyr should not be sprayed under trees, because that tree is likely to be killed, whether or not that was the intention. 

Pesticide Application Notice under willow trees in Glen Canyon Park, December 2011

Much of the ivy that was sprayed by the Natural Areas Program in Glen Park in December 2011 was sprayed under willow trees.  The willow trees are native, so it seems unlikely that they intended to kill them.

Resistance to pesticides

The Federal Drug Administration recently banned some use of antibiotics in domesticated animals because the bacteria antibiotics are intended to kill are developing resistance to the antibiotics.  This resistance is becoming increasingly dangerous to humans who are also the victims of those bacteria.  Antibiotics are being rendered useless by overuse on domesticated animals.  When humans need them, they won’t work because bacteria have developed a resistance to them.

Likewise, plants and animals are also capable of developing resistance to pesticides.  Glyphosate is the most heavily used herbicide in agriculture.  Recent research indicates that weeds are developing resistance to glyphosate

The manufacturer of imazapyr says explicitly that repeated use of this herbicide is likely to result in resistance to it over the long term:  “When herbicides with the same mode of action are used repeatedly over several years to control the same weed species in the same application site, naturally occurring resistant weed biotypes may survive…propagate and become dominant in that site.”   So, does it make sense to use imazapyr on a plant as persistent as ivy? 

The GGNRA reported spending $600,000 over 3 years trying to eradicate ivy from 127 sites.  They were successful in only 7 of the sites.(2)  Obviously eradicating ivy is not a one-shot deal.

If ivy must be eradicated, pesticides do not have to be used to do it.  The Audubon Canyon Ranch in Bolinas Lagoon reported “qualified” success using hand-pulling methods on 5 acres over 5 years “utilizing 2375 volunteer hours.”  Biannual monitoring of resprouts will be required for the foreseeable future.  It’s a big commitment, but at least it is safe. 

All risk, no reward

Congratulations to any reader with the patience to slog through this tedious list of apparently incompetent use of pesticides by San Francisco’s misnamed Natural Areas Program.  We reward your persistence with this summary:

  • Combining pesticides is risky business because the toxicity of such combinations has not been tested.  Therefore, when there is no benefit in doing so, these combinations should be avoided.
  • A pesticide that is appropriate for one purpose is not necessarily appropriate for another.  In this case, imazapyr may not accumulate and persist in a tidal estuary, but it is more likely to do so in a stable watershed.
  • The Natural Areas Program may be killing plants it does not intend to kill by using herbicides indiscriminately.
  • Herbicides should not be used repeatedly on the same plants in the same locations because the plants will develop resistance to those herbicides. 
  • If the Recreation and Park Department is planting ivy in one park and destroying it another, could it be such a bad plant that it is worthwhile to expose the public to toxic pesticides?  We don’t think so, but if we are wrong, then ivy should be removed by hand without using pesticides.

(1) “Aquatic Pesticide Application Plan for the San Francisco Estuary Invasive Spartina Project,” August 2010, page 32.

(2) Liston, Heather, “Reuniting old adversaries can beat back exotic invaders,” California Wild, Winter 2006