Four Things That Kill Overwintering Perennials

Here are the four things that kill overwintering perennials.

The first is too-wet soil. A soil that holds moisture over the winter tends to rot plant crowns. A winter that has an excess of freeze-thawing will create wet soil with a layer of ice over top and this is certain death for many plants.


While I used to keep my mulch over top of all my plants in the winter, I now tend to pull it back from all the crowns at least 8-inches. This space allows the crowns to breathe and lose excessive moisture (as long as there isn’t a layer of ice on the soil). The plants also come up faster in the spring with this clear area around the crowns.

If you have problems with overwintering perennials or if you want to push the gardening zone on perennials, the single most important thing you can do is increase the drainage in the garden or grow those tender perennials in well-drained areas.

Excessive cold will kill tender perennials planted in colder areas. Butterfly bush is an excellent example. It is sold in many garden centers as a hardy perennial but in USDA zone 4, it is marginally hardy.
Buy plants that are hardy in your zone

Old age. Many beginning gardeners don’t understand that the lifespan of a perennial isn’t “forever”. Most perennials live 3-5 years before they go to the great compost heap in the sky. The longer-lived ones (peonies, daylilies, hosta, astilbe) can easily reach 15-20 years but most others are shorter-lived.

Just Because: In my perennial garden, I plan on losing 15% every year just “because” – with no reason other than “it’s dead”.

Combinations. In the nursery trade, we know that winter will kill a different plant every year. Some year, the unique pattern of weather might wipe out Shasta daisies. The next it might be bleeding heart. This won’t happen in every garden but there will be a general regional plant loss that will mystify every garden expert and cause a run on that plant in the garden shops. Plants that are otherwise bone-hardy will suddenly die with no apparent reason. But it sure annoys the heck out of gardeners.

Mulch Is An Excellent Way To Increase Success With Overwintering Perennials

Mulch helps even out the swings in temperature and it also helps hold the snow over top of the plants. You’ll lose more plants in a year with a fluctuating spring (from high to low and up again) when the plants are thawed out than you will in a year where everything stays frozen until it is time to grow. While keeping my plants frozen a little longer with a thick mulch may mean the early spring bloomers are a few days late blooming, I do know they are alive.

One of the greatest “mulches” for tender perennials is to put old Christmas tree branches over top of the very tender plants. This holds the snow in around the plant and keeps them happy until spring. Remove the branches when the snow melts.

 

How To Think About Your Overwintering Perennials In The Spring

Do not give up on a plant until you know the plant. For example, every year I’ll get letters asking why this plant or that plant is dead. And I write back suggesting they wait another few weeks because the plant is either a notoriously late starter (Coreopsis verticillata) or just sulking a little (hibiscus).

I moved my hibiscus to a holding garden in October 05 and then to its garden location in April 06. It finally showed up in early July with two shoots, grew six-feet tall in 6 weeks and started blooming the end of August. Go figure. Even I had given up on this one but it surprised me.

If you’re poking around, you can check to see if the root is soft and mushy (dead) under the mulch or whether the root and eyes (small growing points) are hard (alive). If a lavender or woody plant, you can gently scrape the bark with your thumbnail and if it is bright green under the bark, it is alive. If brown, that branch is dead. Enough dead branches and the plant is dead.

This always assumes that a plant is installed in the right light and soil areas. If you try to grow a plant out of its preferred location, you stress it. If you stress it enough, it will winterkill.

Clay soils that hold moisture will rot out roots of plants that want drainage. Plants that want full sunlight and are planted in part sun will go into the winter weakened.

I note you may get away with this for a few years but eventually, the stress of the growing condition will show itself over the winter.

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The First Bloom For 2022

April 8 and this hellebore or Christmas rose earned first bloomer status as it normally does.

hellebore or Christmas rose in bloom in early April
Hellebore or Christmas Rose in early April

Yeah, the foliage is winter-brown and I haven’t trimmed it off to allow the flower to really shine out. But it’s April 8 and my garden flower season has launched here in USDA 4.

Note in warmer zones than mine, the foliage will not look as bedraggled and dead.

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Four Things That Kill Perennial Flowers Over The Winter

And there are a few things you can do to help Mother Nature

Photo by Galina N on Unsplash

The soil is too wet

A soil that holds excessive moisture over the winter will rot plant crowns. A winter that has a great deal of freeze-thaw cycles will create wet soil with a layer of ice over top and this is certain death for many plants.

While I used to keep my mulch over top of all my plants in the winter, I now pull it back from all the crowns at least 8-inches. This space allows the crowns to breathe and lose excessive moisture (as long as there isn’t a layer of ice on the soil). The plants also come up faster in the spring with this clear area around the crowns.

If you have problems with overwintering perennials or if you want to push the gardening zone on perennials, the single most important thing you can do is increase the drainage in the garden or grow those tender perennials in well-drained areas.

It’s too cold out there

Excessive cold will kill tender perennials planted in colder areas.
Butterfly bush is an excellent example. Many garden centers sell it as a hardy perennial but in USDA zone 4, it is marginally hardy.
Buy plants hardy in your zone

Plants in containers on a deck or in a garage

I remember some research from “back in the day” saying plant roots died at 5F or -15C. So if you’re trying to overwinter a plant in a garage or unheated porch, that’s the temperature you need to avoid. Note small containers also have water issues with the soil drying out plus the soil freezing and this is a double dose of death.

Old age.

Many beginning gardeners don’t understand that the lifespan of a perennial isn’t “forever”. Most perennials live 3–5 years before they go to the great compost heap in the sky. The longer-lived ones (peonies, daylilies, hosta, astilbe) can easily reach 15–20 years but most others are shorter-lived.

In my perennial garden, I plan on losing 15% every year just “because”–with no reason other than “it’s dead”.

Who knows but everybody loses this one

In the nursery trade, we know that winter will kill a different plant every year. Some year, the unique pattern of weather might wipe out Shasta daisies. The next it might be bleeding heart. This won’t happen in every garden but there will be a general regional plant loss that will mystify every garden expert and cause a run on that plant in the garden shops. Plants that are otherwise bone-hardy will suddenly die with no clear reason. But it sure annoys the heck out of gardeners.

What can gardeners do?

Mulch helps even out the swings in temperature and it also helps hold the snow over top of the plants. You’ll lose more plants in a year with a fluctuating spring (from high to low and up again) than you will in a year where everything stays frozen until it is time to grow. While keeping my plants frozen a little longer with a thick mulch may mean the early spring bloomers are a few days late blooming, I know they are alive.

One of the greatest “mulches” for tender perennials is to put old Christmas tree branches over top of the very tender plants. This holds the snow in around the plant and keeps them happy until spring. Remove the branches when the snow melts.

How to think about your perennials in the spring

Do not give up on a plant until you know the plant. For example, every year I’ll get letters asking why this plant or that plant is dead. And I write back suggesting they wait another few weeks because the plant is a notoriously late starter (Coreopsis verticillata) or just sulking a little (Hibiscus).

When I moved, put my Hibiscus in a holding garden in October 05 and then to its garden location in April 06. It finally showed up in early July with two shoots, grew six-feet tall in 6 weeks and started blooming the end of August. Go figure. Even I had given up on this one but it surprised me.

If you’re poking around, you can check to see if the root is soft and mushy (dead) under the mulch or whether the root and eyes (small growing points) are hard (alive). If a lavender or woody plant, you can gently scrape the bark with your thumbnail and if it is bright green under the bark, it is alive. If brown, that branch is dead. Enough dead branches and the plant is dead.

You can get a ton of my gardening tips and information right here

Often it’s something the gardener does…

This always assumes that you put your plant in the right light and soil areas. If you try to grow a plant out of its preferred location, you stress it. If you stress it enough, it will winter-kill. So clay soils that hold moisture will rot out roots of plants that want drainage. Plants that want full sunlight and are planted in part sun will go into the winter weakened. I note you may get away with this for a few years but eventually, the stress of the growing condition will show itself over the winter.

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The Easy Way to Start Your Perennial Flower Garden

There have been a few questions about perennial flower garden bed or soil preparation and maintenance. In short, “how do we do this?”

And like all things in my garden-world, there is a simple way and a tough way to do this kind of thing. I’ve never understood why gardeners go through all the hard-way exercises when Mother Nature does most of the work for you if you’ll let her.  My best guess on this is beginning gardeners think the experts who do all these strange things must be right so they copy them.

Here’s my take. I like easy gardening that works. Many of the way-more-work techniques work faster or slightly better. But I’m an 80:20 kind of gardening guy. I know 20% of the work produces 80% of the results so I’m happy to identify the 20% and focus on doing that well.

The Hard Way

You can take soil tests and adjust the soil pH and fertility based on those tests. This is often recommended by garden writers with more time on their hands than is safe for their readers. You add whatever nutrients the test recommends and you add copious amounts of organic matter into the soil before planting. You create a super soil.

Or, The Easier Way

You can dig and cultivate the soil around plants and add copious amounts of compost every spring. Or, even middling amounts of compost.
or…

Or, The Really Easy Way.

You mulch the perennial flower garden bed. Period. Oh sure, you can apply compost if you have extra but that decomposing mulch is going to work wonders for you.

Gardening isn’t rocket science (unless you like the fiddling around). And if you simply create an environment that supports a “natural” kind of growing system, the plants will take care of themselves (for the most part).

Yeah, it sounds simple but here’s the deal for those of you who care about such things.

An organic mulch composed of old bark, leaves or other material that breaks down replicates what Mother Nature does. it provides a layer of decomposing organic matter at the soil line that supports a massive number of micro-organisms. Do not use inorganic material such as stone or rubber. My own bias is that I also don’t like dyed mulches (they contain dye – a chemical and they don’t look at all natural).
These micro-organisms are responsible for modifying that organic matter and turning it into useable plant food for your perennials.

They are responsible for balancing the soil pH and maintaining it within the limits created by the underlying rock material that made the soil in the first place.
They are responsible for more “things under the sun” than we normally think of – things like eating bad fungi and bacteria that want to attack your plant but get eaten by nematodes in the soil.

Mulch allows roots and worms and other larger soil creatures to loosen up the soil and keep it well aerated the way you want it to be.

Somebody is going to ask so yes, you can simply toss a bit of compost on top of this mulch in the spring or if you’re really bent on feeding the plants, you can toss some organic feed or spray fish emulsion over top of the mulch.

So you mulch your perennial flower garden bed – top it up every fall and maintain a 3-4 inch layer. (Pull back the mulch away from plant crowns so the excess moisture there doesn’t rot the crown and kill the plant over the winter).

You create paths and don’t walk on the soil unless necessary

I remember talking to an orchid expert who was doing research on wild orchid populations in North America and talking about how just putting one foot down next to an orchid would reduce the flowering compared to one that wasn’t walked near. A single step compacted the soil enough so the natural micro-organism chain was disturbed in that specific region for this sensitive plant.

I’m not saying regular garden plants are that sensitive (they aren’t for the most part) but mulch will loosen up the soil and the only thing that will reduce that is if you walk on it.

That pretty much takes care of ongoing soil work. Mulch and don’t walk on the bed any more than necessary.

Initial bed preparation

There are any number of ways to get a garden bed started. From double digging, to tilling to not-a-darn-thing but laying on the mulch.

You need to eliminate weeds before you mulch – that’s the only trick to ongoing maintenance.

In my latest beds, I tilled up the big bed (here’s my roto-tiller review) but hand dug the smaller beds. Then I mulched and am on weed prevention detail now as I plant and maintain.

For ongoing maintenance however, it’s the mulch, don’t walk and weed routine.
I like it when things are simple. And this is the simplest way I know to keep a perennial flower garden bed healthy and productive for many years – with the smallest amount of work.

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Do I Cut Perennial Flowers Back In The Fall?

Photo by Samantha Gades on Unsplash

There are two different points of view on whether to cut back perennial flowers in the fall or whether to leave them alone.

Cut Them Back And Remove Them From The Garden

This leaves the garden neat and tidy going into the winter. All seed heads are put into the compost or shaken off the plants to germinate the following spring.

But your garden does not attract or feed overwintering birds.

Some gardeners do it because they believe it removes any diseased stems/leaves from the garden. In my opinion this is highly overrated as a reason. If you’re paying attention, any serious problems have already been pruned out and removed.

And common fungal problems such as botrytis (the gray fuzz on decaying leaves or black spots on peony leaves) is one of the most common fungi in the world and you won’t ever reduce or eliminate it on susceptible plants.

Leave Perennial Flowers Alone To Winter In The Garden

Doing this allows the seed heads to feed the birds. Any they miss will germinate the following spring.

It’s far less work but it’s not neat and tidy until later in the winter when the stems fall over naturally.

In my opinion, you do whatever seems right – but you do have to do this at some point.

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How To Grow and Propagate Lavatera in the Perennial Garden

I really like my Lavatera blooms and as I wander through my gardens this past week, the several species of Lavatera that I am growing are simply stunning.

Origins

It seems that the 17th century Swiss botanist that this plant was named after, J. R. Lavater has been well remembered in my garden. They are obviously well remembered in their native habitats of the N.W. Himalayan Mountains, Eastern Siberia and even warm Australia.

I really like this plant because even though each individual flower is short lived (a day or two at most) my perennial plants are smothered with newly produced flowers for an extended period of time.

Lavatera cachemiriana
L.cachemiriana Image the author

Perennial and Annual

I note that my perennial lavatera plants are different from the L. trimestris annuals that are regularly available in garden centers. I’ll be listing others over time on the main perennial flower page here.

While the annual forms are interesting, I do prefer the perennials with their bush like growth or tall straight stalks.

However, if you are growing the annual forms, look for:

  • ‘Silver Cup’ a bright pink, dwarf bush form to 30″,
  • ‘Loveliness’ with a trumpet shaped rose flower stretching up to 4′ in height,
  • ‘Mont Blanc’ a dwarf 24″tall, with white blossoms or
  • ‘Pink Beauty’ another 30″ plant with wonderfully large, pale pink blooms.

Issues With Lavatera

All these lovely pink flowers have not come without a price however. The seeds are slow and difficult to germinate and I’ve had to experiment with different species to see if they were hardy

L. arborea, known as the tree mallow, has twice failed quite spectacularly in attempts at overwintering. After having gone into the winter in good shape, two years in a row I was left with a grey-brown pile of slushy roots in my garden the following spring. I guess this plant had a dislike of Eastern Ontario winters.

I think if I were to seek out this plant again, I would winter it indoors or in a greenhouse to atone for my past mistakes and try to keep it alive.

Update 2019. I haven’t even tried to do this again as I no longer have greenhouses or cold frames. Plants survive outside (or not).

Self-sowing Variety

One plant that has more than redeemed the above failure has been the L.cachemiriana that now self sows itself enough to be called a bit of a nuisance.

This native of Kashmir is quite hardy in our garden and seems to thrive on a sunny garden spot

It starts life as a rosette of leaves and then the second year it sends one or two shoots skyward to five or six feet in height covering them with 2 inch wide soft pink blossoms. The petal is 5 lobed and a tremendous mid pink that brightens up the garden.

Right now, it is also trying desperately to colonize underneath my crab apple tree without a great deal of real success in this lower light area. I know it will pop up somewhere else in the garden in a year or two to try to take over that space and I’ll wind up thinning it down to 3 or 4 plants again.

A Star Variety

The real star of the show is the L. thuringiaca or Tree Lavatera.
This would be a hardy shrub if we lived in Southern Ohio or Pennsylvania but in my USDA zone 4 garden it is killed back to the ground every winter

In the spring it sends up lots of shoots to 6 or 7 feet (the older plants now resemble a large forsythia) and covers them in mid-summer with an abundance of the typical 5 lobed flowers.

Flowers Short Lived

Lavatera flowers are short lived as well but with hundreds of buds being produced, I’ll have a huge display of flowers almost until the middle of August.

I now have two forms of this plant in our garden.

  • I have the species and I have a cultivar called ‘Barnsley’. The newer ‘Barnsley’ is a darker pink than the species and it has a red throat. It is quite an attractive flower and well worth the search to find it.
  • L. ‘Aurea’ or Golden Leaved Lavatera is a stunning plant in the spring when the golden leaves first emerge. It fade a bit to a yellowy green at the end of the summer. It has the traditional pink blooms and is quite a stunning plant when it reaches full size.
  • L.’Bredon Springs’ has rose-pink flowers and a soft grey-green foliage that is quite attractive when placed next to darker leaved plants in the perennial border.
  • I grew L. tauricensis for two years before I lost it during a mild wet winter but I have to confess that there was some discussion amongst my friends and I about whether it was mislabelled. I’m looking for a good botanically accurate source for this as well. It shared the same flowering form as the L. thuringiaca and was quite showy in the trial bed. I’ll be using a German source of seed next year to try and obtain the right plant.

All other forms of Lavatera are for a much warmer garden than my USDA zone 4, Eastern Ontario garden.

Starting Seed

To start the seed successfully, use Doug’s outdoor pot method

  • Take an old 8″ nursery pot (size is not important although 6–8″ is ideal) and cut the bottom off the pot.
  • Plant the pot. I liked that line but it really means to sink the pot in the ground so only the rim is showing.
  • Fill the bottomless pot with either the soil from the hole or a good quality potting soil.
  • Sterilize the soil by slowly pouring a kettle of boiling water into the pot.
  • Once cool, plant the seeds and cover with only a quarter inch of potting soil.
  • Tag the pot so you don’t forget what you’ve planted and then forget it until next spring.
  • In May, after the winter of dormancy, you should see tiny Lavatera seedlings popping up with their distinctive maple like leaves. If it doesn’t have a maple tree leaf, it is a weed and can be removed

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Resources to Find Material Mentioned on This Page

Lavatera seeds — caution — some are annuals but lovely nevertheless 🙂

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