salt – The Garden Professors™

Rubbing salt into wounds

Last week I posted about some horticultural disasters I witnessed in the Czech Republic. This week the chamber of horrors is little closer to home; virtually in my back yard to be specific. Our property backs up to US 127, the main North-South through route in our area. It’s a limited access highway with a posted speed of 70 mph, which means an average speed of 82.7 mph. It also means the road is regularly salted whenever it snows during the winter. As a result, some plants along the highway really take a beating, sometimes with some interesting results.

On Friday I received an email from Susan Gruber, our Undergraduate Advisor, who also commutes on US 127. “Hey Bert, Have you seen the pear trees on the east side of NB 127 just south of the Round Lake Road (and Price Rd) exits? Street side fried, shows up great with flowers on the other side. Also same trees planted at the same time doing great by the ramps, farther from the road, slower traffic, some up on the berms etc. Make great GP fodder, but I didn’t have a decent camera or the guts to pull over and do phone photos in rush hour.”

Unaffected pears upslope and away from salt exposure

Indeed I had seen the trees. As Susan noted, the sides of the trees facing the highway were fried, the opposite sides were in full bloom. I got a few photos over the weekend but the effect was a little less striking than earlier in the week when the trees had blooms but hadn’t begun the leaf out.

Pear trees in the line of fire

The planting illustrates a classic example of wrong tree-wrong place. Interestingly there are several crabapples that were installed as part of the same planting project that seem to be doing well. Selecting trees for exposure to deicing salt is a dicey proposition since, like this example, most of our information is anecdotal. Where the effects of deicing salt on plants have been systematically examined, the studies may focus on only soil exposure or only aerial deposition; whereas trees in the real world get it with both barrels.

Eastern white pine after one winter at a rest area along I-96 east of Lansing. An ideal proving ground for salt tolerant plants.

Ironically a colleague of mine in our department and I put together a proposal several years ago for our state Department of Transportation (MDOT) to identify salt tolerant plants for roadside plantings. Our plan was to install a series of replicated plantings of perennials (trees, shrubs, herbaceous perennials) at highway rest areas throughout the state. We would then correlate plant performance with degree salt exposure and identify plants that could survive, grow and maintain their aesthetic value under the highest salt loads. Initial discussions were positive until the proposal worked its way up the chain of command. Finally it was determined – I am not making this up – that MDOT could not participate in a project on indentifying plants that were tolerant of deicing salts because that means they would have to admit that salt was causing a problem. OK; just don’t tell the trees on US 127…

In our studies we rate plants on a scale of 1 (alive) to 5 (dead). These are class 4 (wish they were dead…)

Right Plant, Wrong Place?

It’s (apparently) Dead Tree week here on the GP!

[To my GP colleagues…we should make “Right Plant, Wrong Place” one of our categories, sort of like “Knock it Off.” Or maybe just “Dead Plants in Action.”]

Exhibit A:

Some unhappy little Magnolias. Drip irrigation was running, which leads me to believe salt spray is the culprit.

I was pedaling my sweet Electra beach cruiser down the sidewalk on Cape San Blas, Florida (or Cape San Blarrrgh, if you caught my post last week) and happened upon this tragedy. In the background is natural coastal sand pine scrub including saw palmetto and myrtle oak. Growing on nothing but sand and a bit of decaying organic matter, 300 yards from the ocean, these are tough plants. Pretty darn salt tolerant. Cabbage palms, also very salt tolerant, marched along the sidewalk. Southern magnolias had been planted in between, and they weren’t faring as well.

Just like “deer resistant,” “salt tolerant” is a rather vaguely-defined grouping of plants. You can find list after list on the interwebs and in the back of nursery catalogs and reference books. The source of the information is rarely confirmed, and if so, it’s the same Extension publication that has been cut and pasted to death. Degrees of salt tolerance are further described as “high,” “moderate” and sometimes “low” without any quantitative parameters (a range of soil electrical conductivity perhaps?). There are tremendous scientific resources (including funding) devoted to breeding for salt tolerance of food crops like rice, barley, and soybeans. Ornamental plants are pretty unimportant in the grand scheme of things (like the global food supply) thus the mere smattering of practically unfunded research out there, leaving us with only anecdotal evidence. Though some lists have M. grandiflora listed a “moderately” salt tolerant, I vote to move it to “low.”

A salt bath for your tomatoes?

This morning I got an email from one of my gardening colleagues, wondering about the wisdom of watering tomato plants with salt water. He had a link to a UC Davis website which tacitly endorses spraying tomato plants with 10% salt water “to increase their nutritional value and taste.” Unreferenced “worldwide studies” are mentioned, along with the “major potential benefit of providing irrigation for crops in areas with freshwater restrictions.”

Before we deal with the impracticalities and out-and-out harm of using salt water for irrigation, let’s look at why this practice would work on tomatoes. By training I’m a plant stress physiologist (and I’m well versed in the primary literature on this topic). Watering tomato plants with a salt solution imposes a drought stress on the entire plant, as less water is taken up under these conditions. So leaves and fruits are smaller and they may produce stress-induced biochemical compounds in response. The upshot is that you have smaller tomatoes with a higher concentration of various solutes, some of which might be tastier or more beneficial to humans.

Guess what? You can do the same thing by decreasing irrigation during fruit set! Less water means smaller fruit and increased concentrations of sugars and other plant compounds, and voila! So you can skip that step of adding salt water and just cut back on irrigation to induce a mild drought stress.

So…why in the world would you dump salt water on your garden soil? The article blithely dismisses this: “Many are still concerned about salt causing soil degradation and rendering some seawater-treated tomatoes inedible, but scientists cite that plants thrive in balanced soil containing both macro- and micronutrients.” Sorry, but sodium is NOT a micronutrient for most plants and does NOT contribute to a “balanced soil” in one’s vegetable garden.

An ironic twist to this whole article is that most of the research that’s been done is relevant to arid parts of the world (the Middle East, primarily) where saline soil conditions and limited water are common. I can’t imagine what they would think about people who would deliberately contaminate good soil by adding salt water to it.

Salt solutions

Hopefully everyone got their filling of turkey and dressing over the long Thanksgiving weekend. I used our unusually mild weather on Saturday to celebrate a time-honored tradition around the Cregg farm: The annual cursing of the tangled Christmas lights.

Turning the calendar over to December in Michigan means another Midwest tradition is just around the corner as well: The annual dumping of the road salt. Although totals vary, at least one source estimates that road crews pour 8 million tons of salt on roads in the US each year. The effects of this sodium chloride are readily apparent on our vehicles – in Michigan it’s rare to see a vehicle more than 10 years old on the road – and drivers of those older cars that remain can usually see the road pass under them through a rusted-out floorboard. Of course all this road salt has profound implications for landscape plants as well. Sodium chloride can damage plants in several ways. Both sodium and chloride can cause direct toxicity, particularly from salt-laden spray drift from highways. Salt in soils can cause osmotic stress resulting in drought injury. Sodium in soils can displace potassium, magnesium, and other essential plant elements. High levels of chloride in plant tissue can reduce cold hardiness and make plants more susceptible to freezing injury. Suffice to say that salt is bad for plants.

So what’s a plant lover to do? In general I advise a two-phase strategy of Selection and Protection for homeowners and landscapers that have to deal with heavily salted roads. By Selection I am referring to planting salt tolerant plants (or at least avoiding salt sensitive ones). The poster child for a poor choice in Michigan is eastern white pine, which is extremely intolerant of road salt. Witness this planting at a rest area along I-96 between Lansing and Detroit.

Don’t you love to see your tax dollars at work? Even a cursory look at a list of salt sensitive plants would have been a tip-off that that white pines and highways are a bad mix. Most blog readers should be able to come up with a list of salt tolerant or salt sensitive plants for their area by Googling their way around the internet. A couple caveats about these lists: First, many are based on anecdotal experience, not hard data, so you may see inconsistencies between lists. If possible, try to consult several sources and look for a consensus opinion about the plants you’re interested in. Second, many recommendations are dated and include plants that may be considered invasive or no longer recommended for planting. One list I just looked at included Russian olive (invasive) and green ash (no longer planted in the Midwest due to Emerald ash borer).

Another approach to reducing salt damage to plants is Protection. Here I am referring to erecting a physical barrier to block salt spray or salt splash from reaching plants. The most typical form of barrier around here is a wooden frame or fence covered with burlap or canvas. Some people will actually wrap their evergreen trees or shrubs with burlap. This may help against winter desiccation but does little for salt since the salt-saturated burlap will still be in contact with the foliage. Obviously aesthetics go out the window with the protection approach but I’m noticing more and more people are willing to put up with a couple of months of looking at burlap to keep their plants looking thrifty the rest of the year. In northern Europe some road systems will line their roads with pre-formed plastic barriers to reduce salt splash to adjacent vegetation.

What about alternative deicers? This is a question I get frequently when I speak about salt and plants. There are several things to consider about alternative (i.e., not sodium chloride) deicers. First is the cost. All alternative deicers are more expensive than salt, some by a factor of 10 times or more. This often limits their widespread use for highway departments. Many road crews will use alternative products around bridges and other sensitive areas where they want to limit corrosive damage. Alternative products may also be useful for smaller scale operations such as parking lots and walkways. Secondly, many alternative deicers contain calcium and/or magnesium chloride. These may even be marketed as ‘plant safe’ since they contain calcium and magnesium, which are essential plant elements. The problem is the chloride. Chloride is a plant nutrient but is only needed in minute amounts (a few parts per million). At higher levels it becomes toxic.

A study in Colorado found tree damage due to chloride near roads which were treated with calcium and magnesium chloride for dust abatement. For safer alternative deicers consider chloride-free products such as calcium magnesium acetate.

In the meantime here’s hoping for a mild winter for everyone.