The Winter of Our Plants Discontent

In 2012 the US Department of Agriculture released a new Zone Map that reflects the overall rising temperatures of our times. Where much of middle Tennessee had previously been considered to be in zone 6, the updated map showed our area to now be solidly in zone 7. And the last decade or so of middle Tennessee winters certainly backed that up. Until the endless, grindingly cold winter of 2013-2014, that is. Seasonal Affective Disorder, anyone? But at least we humans have been mostly inside. Our landscapes have suffered through weather they haven’t faced in a long time.

The damage

Those mild winters year after year encouraged gardeners and landscapers alike to put in plants that formerly wouldn’t have been considered appropriate for the area. Zone 7 plants like loropetalums, camellias, aucubas, gardenias and many evergreen daylily varieties have suffered intensely this winter. Even zone 6 evergreens, like the hardiest forms of Southern Magnolia, ‘Nellie R Stevens’ hollies and cherry laurels are showing damage.

gardenia cold damage Feb 2014 web size

 

 

 

 

 

 

 

Gardenia showing cold damage. This is ‘Grif’s Select’, a zone 6 hardy variety. Leaves are dead but the plant’s still alive.

It’s not just that it’s been cold; it’s how the cold happened. After the holidays a really bad combination of events came together to wreak havoc on normally sturdy plants – a long very cold spell with little moisture, followed by a horrible temperature drop accompanied by brutally cold winds. That cold wind is particularly damaging. It sucks the moisture out of leaves and even twigs, and because the ground is frozen the plant can’t draw up moisture to replace it. The result is that brownish burnt look and shriveled twigs, particularly on the side that took the brunt of the wind. The damaged parts of the plant won’t recover from this.

 

Boxwood dessication & cold damage  web size 2-16-14

 

 

 

 

 

 

American Boxwood showing desiccation on exposed topmost foliage from cold winds with a winchill below 0°F.

As we (please!) move into spring, more damage will become obvious. People who like a ‘tidy’ landscape over winter often cut back grasses and perennials in the fall. Landscapers often do their crape murder (whacking back crape myrtles to those deformed knobbly nubs – a widely prevalent but poor practice) and rose and other shrub pruning in December. In mild winters this won’t have negative consequences. In an old-fashioned cold winter, it might. That winter-killed top structure of grasses and perennials insulates the crown of the plant, protecting the buds that make next year’s growth. Shrubs often die back from the pruning point when subsequently exposed to really cold temps. Such injury will become apparent as the plants hopefully kick back into growth.

One other kind of damage I’ve been seeing this winter is mucho vole activity. If one of your shrubs starts to lean over drunkenly, check its roots. Voles can nibble away almost the entire root structure, leaving nothing to hold the plant in the ground. I’ve seen this both with boxwoods and nandinas in January 2014. If too many roots are gone, so is the plant and you‘ll have to replace it. If it still has a fair bit of root mass, you can try to save it if you think it’s worthwhile, but be sure to replant using Vole-Bloc. More on that below.

What to do about the damage?

Mostly, you wait until the weather is turning reliably toward spring – with this exception:

If you have perennials, hit your yard on the first decent day and check that they haven’t been heaved out of the ground by freezing. If this has happened you’ll see the crown/growing point of the plant significantly above ground level, often with some roots exposed to the air. Replant any victims as soon as you can back to the proper depth.  I’ve replanted heaved up heucheras, hostas, daylilies and Russian sage just in the last week.

Other than that, wait. With woody plants, the good news is that total leaf death isn’t necessarily the end of the plant. I’ve seen hollies and gardenias both sprout all new foliage after a bad winter when all leaves were fried. But DON’T go out and prune off dead material until the plant gets active and buds begin to swell. A plant that’s been severely stressed doesn’t need surgery until you know for sure what you’re pruning. If you’re uncertain about whether a twig or branch is dead, scrape just into the bark with your thumbnail – if you see green, it’s alive, no matter how toasted the leaves look.

If your roses were pruned in the fall, expect to have to cut them back again to live wood at the end of the winter – I’ve seen a fair bit of dieback on early pruned roses so far.  I don’t know how the crape myrtles are going to fare; it’s too soon to tell.

Another good reason to be patient is that many plants might look damaged actually are fine, they just have striking ways of dealing with major cold spells that we’re not accustomed to seeing. Rhododendron leaves will go vertical with the edges rolled inward, for example. This is normal and the plant’s just doing its thing. Viburnums tend to look positively hangdog in cold weather, as if their leaves were wilted. Not to worry. Lots of evergreens go very bronzy in the cold, also a normal reaction. They’ll green back up in spring. Nandina drop leaves in the cold, but will reflush with foliage and you’ll never know it happened.

Plants that have suffered some leaf damage, like the magnolias, will drop those damaged leaves over time and be none the worse for wear. Trees in general tend to be pretty rugged regarding climate vagaries.

Is such damage preventable?

In many cases, yes indeed. As mentioned before, if you can resist the urge to clean the dead tops off perennials until late winter, you’ll have less chance of the crowns fatally freezing.  Don’t prune roses until March, and don’t cut on things like hydrangeas until at least February.

The single biggest thing that you can do is water everything really well when a cold snap is forecast. The rule of thumb for nurserymen and gardeners alike is to never send a dry plant into a cold spell. This is particularly true of evergreens, and a good deep watering late in the season can be a lifesaver. That bad weather combo I mentioned up front made such watering tough this winter, because hoses were frozen for a long time *before* the Polar Vortex hit. But plants watered in well earlier in the season came through it pretty well, as opposed to those that were dry.

If you have evergreens that routinely get cold-burned foliage due to wind patterns or their location, you might consider putting up a temporary windbreak. Although not attractive, burlap windbreaks can alleviate much desiccation due to cold winter breezes and save your plants from a lot of stress.

Another method for preventing windburn is to spray evergreens with an anti-desiccant. Anti-desiccants are non-toxic, anti-transpirant solutions that help the leaves retain moisture and keep them from drying out. They’re particularly useful on broad-leaved evergreens. I’ve seen this practice much more commonly further north than here in middle Tennessee, but it would work here, too.  If you ever bought a Christmas tree that seemed to retain its needles longer than normal in your heated dry house, it’s because it was sprayed after harvest with an anti-desiccant.

A good layer of mulch put down as the weather cools in autumn mediates fluctuating soil temperature and moisture levels and helps plants weather the weather better. But do make sure to keep any mulch off perennials and away from the trunks of shrubs and trees. Those devilish voles will use mulch as a cover for doing their evil work on your shrubs. Which brings us to…

Voles and VoleBloc.

There’s a terrific and hilarious blog about defeating voles on this site, which I heartily urge you to read. But here’s the  Cliff-notes version of battling voles: A) Voles are little rodents, also known as meadow mice, and are NOT moles; B) Voles, although they’ll dig their own tunnels, will also use mole runs to get around, so moles often get blamed for vole damage; C) Voles will gnaw the roots and bark off woody plants at or just below the soil line and kill all or part of the plant, and they can make a full-sized hosta disappear in just a few minutes (they really love hostas); D) Voles don’t like to burrow through sharp little rocks; E) Vole-Bloc is sharp little rocks.

If you’ve got voles, plant using VoleBloc. Also called PermaTill, it’s available here at Moore and Moore in bags. I’ve saved roses, nandinas and boxwoods all with VoleBloc. There are some great instructions at their website on how to do this. It really does work, and it’s the only thing I’ve found to be successful if voles have moved into a landscape.

 

Volebloc bag

 

 

 

 

 

 

 

 

 

 

Live and learn.

One possible benefit from a wicked winter like the one we’ve endured is that it can teach us a lot about our property, particularly micro-climates. Walk around and look for patterns of damage and no damage. Where did things get wind burned? Where did they escape unscathed? What plants made it through fine? What bit the dust really fast? What did the voles relish? Through observation we can see where better to locate more tender plants and where only rugged super-hardy types need apply, and maybe what plants to avoid altogether.

It’s a stretch to find a silver lining in all the low, grey clouds of this long winter, but the experience can make us better, more knowledgeable gardeners.

 

 

 

 

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Impatiens – ruh roh

Well, gang, the unfortunate emergence of super bugs and super diseases marches on. You might want to work up a Plan B for that shade bed where you plant impatiens every summer.

Latest in the disease onslaught is Impatiens Downy Mildew (IDM). This fungus-like pathogen attacks our beloved bedding impatiens (Impatiens walleriana), which includes such popular strains as ‘Super Elfin’, ‘Accent’, ‘Dazzler’ and many more, plus the doubles, minis and ‘Butterfly’ types, along with old fashioned Balsam Impatiens and the wild orange jewelweed and yellow touch-me-not. Last summer (2012 season) IDM hit the Nashville area like a ton of bricks. If your impatiens were looking good and then started looking puny, quit blooming, defoliated quickly and were pretty much collapsed and gone in a few weeks, you had IDM. The disease showed up in early summer and spread like wildfire — I’d say 50% of my clients lost their impatiens plantings by mid-August last year. Hot, humid weather promotes the disease, meaning our area is just as perfect for growing Impatiens Downy Mildew as it is for growing impatiens. Aren’t we lucky.

So what is it and where did it come from?

Impatiens Downy Mildew has been around for a long time, but it hasn’t been a virulent plant-killer until just the last few years. Scientists don’t know yet whether the original disease has recently mutated into this much worse form, or whether this is a new closely related disease that jumped species from some other host into the impatiens family. IDM showed up first in Europe about three years ago, and over across the pond gardeners have cut back significantly on their impatiens plantings due to the widespread destructiveness of the disease. 

In the US, the first reports of IPD were from Florida. It then spread north up the coast, west along the Gulf Coast and up to the Midwest. It’s even shown up in California; thirty states reported IDM outbreaks in 2012. Many of our bedding plants come out of Florida, particularly seedling plugs that ship to finishing growers all over the country, which is one possible way the disease was initially and inadvertently spread.  Needless to say, professional growers recognized this new disease threat very quickly and jumped to find a rotation of fungicides in their greenhouses that would prevent infection. This rapid response has served to significantly clean up the plants coming out of the big growing outfits, so what arrives at garden centers like Moore & Moore is a good bet to be disease free. But unfortunately it doesn’t mean the plants won’t get a dose once they’re planted in the landscape. I’d say it’s actually likely plants will get infected after planting at this point.

Don’t worry about your other garden plants, though. Although Downy Mildew looks the same on lots of different types of plants, there are actually many different pathogens that produce a disease we lump under the name Downy Mildew, and each one is pretty host-specific. What this means is that Impatiens Downy Mildew only affects impatiens, and you don’t have to worry about it spreading around to other kinds of plants in your garden.

How does it spread?

In the garden, IDM spores are easily spread by splashing water, particularly at night (which is, of course, when irrigation systems are usually timed to kick on). But how did those spores reach the garden in the first place, assuming you put in clean, disease-free plants? Evidently they float in on the wind, folks. It’s unclear how far spores can travel, but they’re incredibly tiny – dust mites, if you will – and a tropical system could theoretically dump billions of IDM spores up the entire East Coast. It’s quite possible one did.

And more bad news: plant pathologists are pretty sure that the spores can also overwinter in the soil of the infected bed, so to avoid almost certain disaster DON’T plant impatiens in a bed where you had mysterious plant decline and death last summer.

What are the symptoms of Impatiens Downy Mildew?

I’ve nabbed some pictures off public domain horticultural websites to show you the progression of the disease (believe me, every hort school in the country, not to mention growers, breeders and chemical companies are all over this topic). As mentioned above, the entire progression takes maybe 3 to 4 weeks in typical summer weather.

The first shot shows a fairly early stage of infestation. You can see leaf yellowing, plus curling, wilting and tip death on newer growth.

Picture 2: If you flip over a yellowed leaf and look at the underside, a blanket of white will be found – these are the fruiting bodies that release the spores (the “downy mildew”).