Overwintering tender plants,
Fall leaves over mulch,
Dwarf Alberta spruce reverts,
Prune roots of trees you're keeping
Vertical mulching for tree roots'
Dahlia and Canna storage,
Fall lawn care,
Fertilizer, and sulfur for rhodies,
Nixing the heavy footed holiday
ahead for gardener who wants to winter tender plants
I'm in zone 5 but from what I've seen over the years I'm
in a microclimate so I have a lot of zone 6 plants doing well here,
Lately I've been looking and thinking about all the
plants I want to bring inside or somehow keep for next year. I
don't think I have room for all of them. What I want is some kind
of thermal blanket I can lay out over things to keep, say, zone 7
plants alive through winter. Or maybe would a portable greenhouse
set over them do the trick? Or a greenhouse with a small heater? -
You can certainly try it. Perennial growers have worked out some
of the details for you.
Many growers have to deal with hardiness issues, even though
they grow plants hardy to their area. That's because roots are less
hardy than plant tops. Roots lose even more of their cold tolerance
when they're within a pot, above ground, not insulated by soil as
they would be if they were growing in the ground. Like plants in a
patio container, they can lose a whole zone, hardiness-wise. That
is, it might take a zone 4 plant to survive a zone 5 winter in a
pot above ground.
Some growers "buy back" that zone this way: They clip back their
potted stock once hard frosts are occurring regularly, set the pots
in tight rows on flat ground, then put a layer of insulation over
them and follow that up with an opaque white plastic cover. They
lay heavy boards over all edges of the plastic to seal it to the
ground. This may keep the plants' roots ten degrees warmer -- a
zone's worth of insurance.
Unfortunately, that space is now attractive to animals that live
outdoors. It's warmer and there is food -- plant roots. Mice that
get into this shelter can do a great deal of damage. So before the
insulation goes over the pots, poison bait is placed throughout the
The same would be true of a portable greenhouse, or as we should
call such a structure if it's only minimally heated, a cold frame.
It would become a vacation destination for rodents and more. So be
Is all this practical? Even if you use a small heater and set it
to keep the cold frame just above freezing, it could be an
expensive proposition. And given the low insulation factor of
transparent plastic walls plus the effect of sunlight on the
interior, dealing with uneven temperatures inside the structure
could be quite the headache. Plants in one corner may freeze solid
and die while those closest to the heater or the ground might
survive. Others warmed most by winter sun would die as a result of
the huge temperature swing they'd feel each day at sunset. Those
susceptible to botrytis and other fungal problems might become
hotbeds for their diseases unless the frame can be ventilated on
The lower the cold frame, the better, so it can make the most of
ground warmth. I'd put an opaque cover on it to prevent daytime
overheating, check interior temperature and humidity on mild winter
days and air it out as necessary.
It might be simpler to dig a root cellar, even a temporary one.
There you can put your tender plants below the frost line. Stay
tuned for more on that later this month, when it's time for me to
bury my rose trees!
Leaves on top of mulch?
I have a question relating to an article you wrote last
fall about using leaves on top of woody mulch. I use cedar mulch on
my perennial beds. I understand that leaves break down to some
degree, but not all the way, and have mineral properties to nourish
the soil underneath. My question is, might the leaves left on top
of the mulch that don't break down encourage slugs to feast on the
plant or insects to do the same? - V.F. -
Two things. First, there's no certain answer. Only a
The mulch you keep on a bed can create pest problems. Any mulch
can do that, as all organic matter can attract and then support
slugs plus all the other life forms large and small that are
responsible for decomposition. Whether those organisms in that
group that are able to dine on living tissue as well as dead tissue
do indeed make that switch depends on what the living conditions in
the area are when the supply of their preferred food -- dead tissue
-- runs out.
If by adding leaves to a bed you create conditions that allow
slugs or any other component of the decomposition community to
multiply inordinately, you may have problems the next year. For
instance, I'd be expecting a problem if the bed and its persistent
cedar chips was in a condition or maintained in a way so that it
stayed damp. Slugs breed freely where it's damp.
If the bed and its mulch blanket drains well enough and is
irrigated appropriately so that it dries down between waterings, we
would not expect such trouble.
Another possibility is if the worm population has been depressed
in an area, perhaps because insecticides have been routinely
applied to the lawn, slugs might gain the upper hand when presented
with a ton of extra food. If worms were present, they would be
mighty competitors for the fallen leaves and slugs might not
If you have a well balanced natural community of decomposers,
the annual leafy bounty wouldn't upset the apple cart. Your slug
problem or lack of one would remain about the same year to
What makes for a well balanced community? One major thing is
soil conditions close to or the same as they would be if the area
was "wild." That includes a regular cycle of fallen debris and
decomposition. So an area kept sterile and bare that was suddenly
heaped with lots of leafy mulch might in the first year see an
explosion of one or another species in the decomposing
Next, V.F., some points that need clarification.
Mulch make up
You write that you use cedar mulch on your perennial beds. From
the article you referenced (Growing
Concerns 584), "...mulches best for perennials are leaves,
grass clips, cocoa hulls or finely-ground bark." You're using a
mulch that's problematic from the get-go..
Our objections to wood as a mulch, and cedar or eucalyptus in
particular, is that it does not break down at a rate that serves
perennials' needs. Wood mulch doesn't create soil conditions like
those perennials would have if their own foliage and stems were
allowed to regularly recycle into the soil. Wood mulches also
remain in relatively large chunks, offering many cavities that
moisture-loving, shade-seeking critters like slugs can call
You also write that you "...understand that leaves break down to
some degree, but not all the way.." Perhaps that's because what we
wrote in that same Growing
Concerns 584 was, "You can leave the leaves as a
mulch or an addition to a mulch. ...whole or shredded. A layer that
is four or more inches deep in fall will be just about broken down
by late spring. However, sitting on top of dry woody mulch, leaves
may not break down well over winter or at the beginning of the next
season. For this, too (as for the assimilation of fertilizer),
moisture is essential."
Perhaps that mis-led you. Sorry! All leaves, even those that
rest on top of wood chips, do eventually break down, entirely. Our
focus was on the person who had written wondering if she could
spread fertilizer and leaves on top of old wood mulch. For that
angle, it was pertinent that leaves break down faster in some
situations than others.
If organic matter didn't break down completely, we would be
quickly buried in an accumulation of shed plant parts, insect
bodies, animal wastes and corpses. Fortunately, this isn't the
case. Nature is very efficient. When we spread four inches of
leaves on a garden, only a paper thin layer remains in April and
that will vanish completely by midsummer.
Beneath the skeletonized leaf parts is a layer of dark crumbly
matter -- worm and mollusk droppings. That will break down even
further and melt into the soil to be taken up by plants' roots.
I have seen a look I want to duplicate, but don't know
the type of ornamental grass used. Each clump along a picket fence
was about six feet tall and they were each tied with a raffia bow.
The plumes on top were beautiful. Can you give me types that would
fit? It doesn't matter the color (these were green with fuzzy cream
tops). I just liked the overall look. I know nothing of ornamental
grasses but I'm suddenly drawn to them. The area where they would
be planted gets full sun. - L.H. -
Maiden grass (Miscanthus species; left) and pampas
grass (Cortaderia selloana, right) are both attractive in
fall for their plume-like seed heads. Some Miscanthus are
hardy to USDA hardiness zone 4, but most only to zone 5.
Cortaderia selloana is barely reliable even in zone 6.
Zone 6 minimum average winter temperature 0° to -10°F and this
plant is often killed at 15°F.
Maiden grass (Miscanthus species) might work, although
if this is what you saw they must have been young clumps to have
been a visual fit for little bow-ties. I think maiden grass bears a
particularly ironic plant name, in that these plants begin as
graceful willowy clumps but mature over the years into
Pampas grasss is a more likely possibility, based on your
description of the creamy plumes. We treat it as a tender species
to be grown as an annual in zone 5 where many of our readers
Some problems have no solution. We have more humor than help to
offer when you pose a "stumper" such as:
Why would a diminutive dwarf Alberta spruce (Picea
glauca 'Conica') think that it can support a full-sized white
spruce (Picea glauca) from the tip of its relatively
skinny trunk? Mine has sported this growth that I think must be a
throw-back to the parent plant and I guess I have to cut it
All too common: A dwarf Alberta spruce reverting to produce full
size branches. Eventually the new growth will become too heavy for
the dwarf trunk at the base.
Yes, that is a non-dwarf limb popping out of that dwarf conifer.
It's a fairly common aspiration of dwarf Alberta spruces, to revert
in this way to the growth habits of the parent species.
Perhaps they yearn to become full sized white spruces. So
perhaps these sadly overused but worthy little shrubs would
appreciate some special distinction. That is, if you let that
reverted shoot grow for about a year before you remove it, its
antler-like outline can make the beginning of a reindeer, devil or
It could be a worthy entrant when we announce a Distinguish
a Dwarf contest next Halloween...
This week in our gardens
Grow with us! This week:
trees in preparation for transplanting next year. Trench about
one-third of the way around each, at the edge of the root ball
you're readying to move. Cut every root in that area cleanly, then
backfill the trench with loose soil and cover it with mulch. Keep
that area moist, and new shoots will grow from the cut roots, even
After the move, those vigorous new tips will do more good for
the tree than raw cut roots.
Vertical mulch: Aerate tree
Fire up the heavy duty gas-powered drill we borrowed from Forum
Moderator Deb Hall
and help a neighbor vertical mulch his very stressed sugar
Neighbors' trees are as important to us as our own, since it's
often someone else's tree that's filtering the wind that would
reach us, muffling the nearby highway noises and moderating the
temperature in our yard. Those trees also shelter birds we love to
hear and provide food for certain butterfly caterpillars we enjoy
seeing but our own plant collection won't support.
So we're willing to help out when a neighbor is willing to do
the right thing by a tree.
We'll give this ailing maple a boost by drilling a 2 inch
diameter hole 12-18 inches deep into the lawn, every two feet
throughout the area covered by this tree's branches. We'll fill
each hole with compost and slow release fertilizer. This 2x2 grid
of holes will help aerate the soil and allow us to place nutrients
where they won't be commandeered by the greedy, efficient lawn
roots before the tree can use them.
turning the canna and dahlia root clumps
that are drying in the garage, so excess soil will fall away.
That will lighten and shrink these big, heavy clumps. Since they
store best whole -- they shouldn't be cut up until spring -- and
most of us have limited cold-storage space, everything that flakes
away is to our benefit.
Bid adieu to the jack o'lantern season by chopping up withering
pumpkins and putting them on the compost.
Image DSC 1256.jpg; Caption: We love a carved
pumpkin, and hate to see the season end. Something about the
ephemeral nature of this art intrigues us. This scene, which Janet
carved and titled "Monster from the deep destroys Lanternville,"
isn't something we would want in our garden all year, but it sure
was fun for a while!
Renew that lawn
Rake away the dead grass, loosen the soil and spread some grass
seed over bare spots on a lawn that succumbed to drought stress
this summer. It's late for sowing a lawn, yet the seed may still
sprout during Indian summer and get a jump on next season.
If lawn seed doesn't germinate before the weather turns too
cold, it will remain there in waiting and start up at the first
possible moment next spring. That's if it's in good contact with
soil and not on a severe slope where rain could wash it away.
release organic fertilizer
on beds to rest moist under and between fallen leaves. It will
start breaking down now and its component nutrients will be there
to boost plants' spring growth, something that begins long before
we resume gardening.
Green thumbs up
To spreading a cup of soil sulfur and some acidic slow release
fertilizer such as cottonseed meal around every
Rhododendron (including those known as azaleas), then
mulching with pine needles, coffee grounds or cocoa hulls. Other
acid loving plants such as Japanese andromeda (Pieris
japonica) and mountain laurel (Kalmia latifolia) also
appreciate this treatment.
However, don't expect it to cause the shrub to bloom. It can
help a budded plant stay healthy enough to keep those flower buds
through winter but can't make buds form before the next bloom time.
Flower formation begins in spring after the shrub blooms and is
supported by the previous year's fertilizer and sulfur
To trampling your gardens and shrub beds as you
put up holiday decorations.
That traffic can have a lasting, detrimental effect on plant
growth. Your foot can put as many or more pounds per square inch of
pressure on the soil as a piece of heavy machinery, closing air
spaces in the ground and depriving roots there of essential oxygen.
Cover your working paths with bagged leaves or bundles of
newspaper. Walking on them will distribute your weight over a wider
area. Or perhaps you can wear snowshoes and really make the
neighbors talk about "that wacky gardener!"