Hedge and Black Walnut Questions and Answers

This week my reaction and comments to two reader emails that I received over the summer. One of them may have solved a gardening mystery that I’ve had on my property for nearly two decades.

The first email came from a reader who was having problems with their privet hedge. The symptoms were that the leaves were discoloring and instead of being a solid green privacy hedge the privet had turned into a yellowing line of pruned shrubs that were unsightly. The hedge owner noted that the problem seemed to become worse over the summer and that it also seemed to be spreading from one neighbor’s hedge to the next until it reached this property.

An arborist was called in (Big Tree) and seemed to confirm my speculation that there was a mite issue, although we disagreed on the variety of mite. That didn’t matter much since my recommendation was to treat the hedge with a horticultural oil like cold-pressed neem oil. Several applications, maybe two at two- to three-week intervals, was my suggestion, but the arborist wanted to use a chemical-based solution.

There was, however, a second possibility that came to mind as I looked over at neighbor Nancy’s 2-year-old privet hedge. She had noticed that the 90-foot-long hedge was clearly yellowing at one end and not the other. Was there a soil issue from when the hedge was first installed or something more sinister going on? It didn’t seem likely that it was an insect issue, and I found no signs of insect or disease.

Quite coincidentally I was watching a program on commercial farming issues where crops performed poorly in some of the locations on one farm and not another. There was no indication that it was a nutrient, insect or disease issue, but when aerial photography from a drone was overlayed on a ground contour map it became pretty clear that there was a drainage issue and at every spot where the crops were doing poorly there was a corresponding but slight drop in the elevation.

In the case of the farm the issue was found to be that water was accumulating and staying in the depressed areas longer than the more rapidly draining balance of the farm where the drainage was nearly perfect. The excess moisture was found to be the problem. The following year when the land was tilled and planted, adjustments were made, and that season the harvest was very even across the farm. Out here on the East End we rarely consider drainage issues, but where there are depressions and underlying deposits of clay this really needs to be considered.

Which brings me back to neighbor Nancy. It dawned on me that there was a clear difference in elevation from one end of her privet row to the other end of the row that was allowing water to accumulate and puddle at the same spot where the privet foliage was yellowing. The clay in the soil at that end as well as the downward slope of the hedgerow was and is probably resulting in water being trapped in the soil at that end, creating a somewhat anaerobic condition (lack of oxygen and air) and the yellowing of the foliage.

That problem solved, at least in my mind, I received another email from the hedge owner in Southampton saying that even after spraying several times her hedge was looking no better and possibly worse. But, and this was a good clue, none of the neighboring hedges were looking any better either, and those had been sprayed all summer with who knows what.

Let’s take a step back though before I tell you what I told the homeowner. There are several diseases and insects that can affect privet hedges. In the Hamptons and in other areas where these hedges go on for miles and miles — and each property’s hedge touches or comes within inches of the next property’s hedge — diseases and insects simply and naturally follow the hedgerow, spreading the infection. It’s the issue of a monoculture (all the plants being the same) and the ease at which insects and pathogens can take advantage of this proximity.

Add to the fact that most of the privet hedges seem to be treated with chemicals as opposed to botanicals and organics and you get resistance built up to the materials being used and the chemicals become ineffective. This is common in both agriculture and horticulture, and for this reason only using one chemical for a short period of time then switching to a different chemistry is critically important. This rarely happens when oils are used as the oils don’t directly affect the insect or disease chemistry. Instead, the effect is more mechanical — the oil products smother the insect or pathogen.

But no matter which was used in the case of the privet, chemical or oil/organics, the stippling and or yellowing of the foliage wasn’t going to look any different this summer no matter how much, how often or what chemistry was being used. This is because of the way the mites feed, and while the mites may be long dead, their damage lives on.

For sucking insects, like mites, a needle-like mouth part called a proboscis is used to extract the juices (mostly carbohydrates) from the leaves, and this is what the mites feed on. Each time the proboscis is inserted and a cell within the plant depleted, a tiny yellow speck is left behind. This is the yellowing or stippling that we see and use as a telltale indicator.

There’s a natural response to think that by feeding the shrub, in this case the privet, that the yellow spots will disappear but when the hedge is fed and if the mites are not under control this can lead to a flush of new growth, not regrowth, that the mites move on for their next feast.

A second email from a reader was about black walnut trees and what you could grow around and under them. My response was this link, and if you have a black walnut on or near your property you may find this very helpful: tinyurl.com/5n8f2npt.

Not familiar with the black walnut issues? The roots of this tree, as well as the husks of the nuts and to a lesser extent the leaves, contain a natural herbicide. This defense mechanism is found in many plants and is referred to as allelopathy. You’ll also find that garlic mustard, the invasive weed, also is allelopathic.

Now I have spots on my property where I often find it hard to grow and establish certain plants or they simply fail to thrive. I never considered that black walnuts could be the issue because I don’t have one on the property. Ah, but 200 feet away on the next street over is a towering black walnut that every squirrel in the neighborhood has an intimate relationship with. But at this distance how could this tree be affecting my plantings?

A gray squirrel sitting on a rock outside our living room window may have given me an answer. The squirrel sat atop a large rock with a tennis ball-sized green, black walnut firmly held between its forelegs. With its sharp teeth it systematically stripped the fleshy covering off the nut leaving a pile of cast off walnut shavings on the lawn. Once it had extracted the nut it scurried to another property where the nut got buried for a winter feast.

The nut itself does not seem to contain juglone, the herbicidal material. This is mostly concentrated in the roots, and that’s why few plants grow under the canopy and root zone of the tree itself. But the second highest concentration is within the nut covering — that was now deposited on the lawn. Now I began to wonder how many piles of nut shavings I’ve seen in the gardens and lawn, and a dim light bulb went off in my head. Was this the reason why I had small spots on the property where things wouldn’t grow or thrive?

I don’t know yet if juglone is water soluble and if it leaches from the nut husks, but I have a strong suspicion, and now when I see the piles of green husks that turn brown once left behind, I think I’m going to start removing the piles and find a way to get them somewhere where everything will be safe from the juglone. The compost pile would be the worst idea since I don’t know the “shelf life” of the juglone, and the last thing I want to do is make my compost gold deadly.

An interesting dilemma indeed. Do your neighbors have black walnut trees? Are squirrels transporting the nuts to your property so they can extract the nuts leaving behind a potentially toxic, though small mess? Maybe something to consider? Black walnut trees were once widely planted for their nuts as well as their highly valuable wood. Food for thought. Keep growing.