Unexpected Results from Too Much Neem Oil

[Gabler]

After a nearly-disastrous run-in with pine sawflies last summer, I purchased some bulk cold-pressed neem oil this spring to spray as a prophylactic. It has also been working well as a preventative for the rust that bounces back and forth between my eastern red cedar and hawthorn. Neither has shown any symptoms since I began spraying, though it could also be the lingering effects of the Daconil I used for two treatments when an orange alien gall showed up in early March.

I started with a pre-mixed spray bottle of neem oil, and I sprayed with excellent results. When that ran out, I diluted some straight, cold-pressed neem oil with water using a squirt of Dawn dish detergent as an emulsifier. Almost all of my trees are still in development, so they've been growing vigorously, and I haven't worried too much about creating "ideal" conditions for those trees, given how healthy they are when allowed to grow unrestrained. Good enough has been good enough. Accordingly, I didn't bother measuring out the neem oil to the exact specifications on the bottle; I just eyeballed it.

When I sprayed my trees, everything looked normal for the next few days, except the trees never looked completely dry after I sprayed them. The oil coated the leaves causing them to appear perpetually damp. Subsequently, some of my trees appear to have suffered from sunburn (or is it chemical a chemical burn?), with brown patches along creases in the leaves where more solution would have settled on the leaf. I can't be certain it was the neem oil that did it, but just as tanning oil increases sun exposure on the skin, I suspect the excess neem oil increased sun exposure to the plant leaves. Where normally leaves are a little bit reflective like glossy paper, they were a deep matte green, without any shine. I suspect the mechanism is similar to oil on a napkin. When the napkin is dry, it's opaque. Use it to clean up some spilled olive oil, and you can see through it.

Since I'm still a novice compared to the lot of you, I could be mistaken, so I'll post some pictures (to follow this post) so you can judge for yourselves and weigh in. You'll see that it has most effected those leaves exposed to the most direct sunlight, though it is also possible that these are the same leaves which were exposed to the most spray from above.

 

[Clicio]

Any oil in excess, even diluted in water, can fry leaves and needles under a hot sun.
it has happened before.

 

[Gabler]

It had the most pronounced effect on this red maple seedling. New leaves are beginning to emerge healthy.

 

[Gabler]

Hardly affected my ironwood trees (Carpinus caroliniana) at all.

 

[Gabler]

Eastern red cedar is unfazed.

 

[Gabler]

The same goes for the Washington hawthorn. It and the cedar got the heaviest application, but suffered the least.

 

[Gabler]

Sweetgum (Liquidambar styraciflua) seems to have hated it, but some leaves were affected more than others. You can see by these two big leaves in the middle of the picture that the left is heavily burnt, while the right is largely unaffected.

 

[Gabler]

My mulberry hybrid (Morus rubra x alba) wasn’t bothered.

 

[Gabler]

Nor was my blackhaw (Viburnum rufidulum) bothered much.

 

[Gabler]

My beeches and oaks didn’t begin extending their leaves until after I sprayed, so they were not affected at all.

 

[Gabler]

Needless to say, I’ll wait a while before spraying again, and when I do, I’ll use a fraction of my prior concentration of neem oil. In the mean time, I figure this is a good case study on sunburn, comparing various species’ response to a combination of oil and sun. I’ll also know to keep my beeches shaded if I ever do spray them. I dealt with sunburn last year just from putting a tree on a metal table in part shade.

 

[rockm]

I'd think since you're applying an oil, this has more to do with that oil blocking respiration on the leaves surfaces. Oil will clog stomata. Since the leaves were pretty young when you applied, they're probably extra sensitive to this kind of thing. Sunburn probably also played a role, but I suspect the blocking was more of a factor in the damage.

 

[Pitoon]

When I spray Neem oil I will move the plants into the shade for a day or two to prevent the leaves/needles from getting burned. Enough time for it to work, then I would spray them down especially the junipers and move them back out into the sun.

You don't have to spray down if using as a dormant spray oil.

 

[Clicio]

I'd think since you're applying an oil, this has more to do with that oil blocking respiration on the leaves surfaces. Oil will clog stomata. Since the leaves were pretty young when you applied, they're probably extra sensitive to this kind of thing. Sunburn probably also played a role, but I suspect the blocking was more of a factor in the damage.

Very interesting, yes, it makes total sense.
Clogged stomata.

 

[Wires_Guy_wires]

I'd think since you're applying an oil, this has more to do with that oil blocking respiration on the leaves surfaces. Oil will clog stomata. Since the leaves were pretty young when you applied, they're probably extra sensitive to this kind of thing. Sunburn probably also played a role, but I suspect the blocking was more of a factor in the damage.

I think this is true albeit just part of the problem. Stomata are mechanical, so any oil droplets will be pinched off like a microscopic turd.
But it's probable! Because we use surfactants that make the oil emulsifiable in water. Good call!

Another thing to keep in mind is that chemistry has a rule that most carbon-hydrogen materials can be dissolved in carbon-hydrogen liquids. I think the original phrasing was something like: polar solvents can dissolve polar molecules and apolar solvents can dissolve apolar molecules.
This means in general that oil can dissolve wax, like the cuticle that protects our foliage from sun damage.
Neemoil, rapeseed oil (used in some insecticides) and olive oil or sunflower oil all can do this kind of damage.
The cuticle should form a waxy film that scatters sunlight. If it's dissolved in oil, it loses that property and sunlight passes through it. On top of the respiration issues and probable plugging of stomata.

In a few days the oil will oxidize and turn dull/matte, this will scatter the light again and the plants can go back to their regular behavior. I don't know if there's a way of speeding up that process.

 

[Dav4]

On a bit of a tangent, but does neem even prevent sawfly larvae if applied in spring?? In my experience, oils are best used in the dormant season (they can be used during the growing season at a weaker dilution and with precautions) to smother overwinter pests.. maybe a better time to prevent Sawfly? Sawfly are particularly robust and I wouldn't think neem oil would bother them at all unless you were out there to literally soak them in it, at which point a blast of water from the hose would be more effective (I like squishing them- very satisfying). I'd look into good systemics for sawfly.

 

[Gabler]

On a bit of a tangent, but does neem even prevent sawfly larvae if applied in spring?? In my experience, oils are best used in the dormant season (they can be used during the growing season at a weaker dilution and with precautions) to smother overwinter pests.. maybe a better time to prevent Sawfly? Sawfly are particularly robust and I wouldn't think neem oil would bother them at all unless you were out there to literally soak them in it, at which point a blast of water from the hose would be more effective (I like squishing them- very satisfying). I'd look into good systemics for sawfly.

Cold-pressed neem oil is supposed to disrupt insects’ neuroregulators or something like that, and they forget to eat and starve. That’s what I’m told, anyway. We’ll see if it actually works. If not, there’s always permethrin.