Oaks and Herbicide

Last spring I noticed an unusual phenomenon in the deciduous oaks when I went to our arboretum in October (yes, spring, the arboretum is in Argentina, so for us, October is the cruellest month). In almost all these trees, the new leaves were twisted into grotesque forms. The effect could also be seen on some shrubs in the garden. After pursuing various theories as to what could be the cause (virus, excess water followed by heat wave, etc.) we eventually focused on the most likely culprit: 2,4-Dichlorophenoxyacetic acid, or 2,4-D for short. The arboretum is surrounded by farmland where crops are planted using the direct-drilling or no-till method, which relies heavily on herbicides, including 2,4-D. The air can never be perfectly still during application, so a certain amount of drift is inevitable. I had noticed this effect in isolated oaks before, but never to such an extent and so consistently on all deciduous oaks. The reason it was so bad this season may be related to the heavy rainfall in late winter/early spring (responsible for the flooding describing in another blog post here). The rains and floods delayed the sowing of crops and hence the application of herbicide, and so 2,4-D was in the air at the time that oaks were leafing out, rather than earlier in the season when they are still dormant. Moreover, this herbicide is extremely volatile, and when it is applied when temeperatures are higher—in early spring, rather than winter—more of it will drift as fumes than is normally the case. So, double whammy. The new growth appears to be extremely susceptible to the herbicide, as the photos below reveal (evergreen species, with more robust leaves, don’t appear to be affected at all).

Quercus acutissima - note contrast between leaves that were tender when hit by herbicide, and those that emerged afterwards.

2,4-D was one of the ingredients of the notorious Agent Orange of Vietnam War fame, or rather, infamy, and is in fact a synthetic auxin, a plant growth hormone. It is absorbed through the leaves and transported to the meristems of the plant, provoking uncontrolled, unsustainable growth, which results in leaf withering and stem curl-over and eventual plant death. It is particularly useful in farming because it only affects dicotyledons or broadleaf weeds, leaving monocotyledons (grasses, cereals) undamaged. Fortunately, in young oaks it does not cause permanent damage, and in fact the second growth flush emerged unaffected, as can be seen in some of the photos (click on them to get a closer look).  

Below are photos of some familiar oaks that may be difficult to recognize! Thankfully the inconvenience is more of an aesthetic nature than a biological one: while growth may be affected this season, lasting harm is unlikely. Our arboretum coexists happily with farming and its side effects (if there were no farm, there would be no arboretum, simple as that) and there are more important threats to worry about. Having said that, 2014 was certainly an annus horribilis with us. Aside from the flooding and the herbicide, we had what appeared to be vandalism in the nursery where we found new growth on precious cultivars snipped off with a clean diagonal cut, similar to that made by secateurs, with the new leaves left where they dropped. All sorts of suspicions were aroused and we even put a padlock on the nursery gate—till someone with more experience informed us that the vandals were probably parrots, whose incisive beaks can cut tender stems like shears. So we can add them to the list of our known quercicides, together with Argentine ants and hares. And on top of that there was damaged suffered by my Q. humboldtii and Q. insignis seedlings at home in Uruguay, at the hands of grasshoppers—though given the biblical dimensions of this season’s calamities, perhaps it would be more appropriate to refer to them as locusts!

Q. aliena var. acutiserrata
Q. arkansana
Q. falcata
Q. hemisphaerica
Q. leucotrichophora (Syn. Q. oblongata)
Q. nigra
Q. muehlenbergii
Q. robur Q. 'Vilmoriniana'
Q. 'Vilmoriniana' Q. rugosa