Grow 640: Overwinter tender, fall mulch, slugs, dwarf reverts, fertilize, roots, holiday lights

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Wonder what's going on? See "Prune roots" 

Fall topics:

Overwintering tender plants,
Fall leaves over mulch,
Mulch/slug matters,
Ornamental grass,
Dwarf Alberta spruce reverts,
Prune roots of trees you're keeping small,
Vertical mulching for tree roots' health,
Dahlia and Canna storage,
Pumpkin recycling,
Fall lawn care,
Fertilizer, and sulfur for rhodies, and
Nixing the heavy footed holiday lights artist

Tough times ahead for gardener who wants to winter tender plants outdoors

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, too.

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? - J.M. -


Dear J.M.,

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 area.

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 prepared.

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 sunny days.

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. -


Dear V.F.,

Two things. First, there's no certain answer. Only a "maybe."

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 proliferate.

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 year.

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 community.

Next, V.F., some points that need clarification.

Mulch make up matters

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 home.

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.


Ornamental grass

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. -


Dear L.H.,

G640Miscanthus7024.jpg           G640Pampas5020.jpg

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 stout-waisted matrons.

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 garden.


Dwarf Alberta Stumper

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 out.


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 Viking costume.

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:

Root prune

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 this fall.

After the move, those vigorous new tips will do more good for the tree than raw cut roots.


Vertical mulch: Aerate tree roots

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 maple.

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.


Keep 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.


Recycle pumpkins

Bid adieu to the jack o'lantern season by chopping up withering pumpkins and putting them on the compost.

G640Pumpkins1256.jpgImage 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.


Spread slow 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 applications.

Green thumbs down

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!"