“Add gravel to the bottom of containers to improve drainage”

Sure sounds good, and is easy to understand from an intuitive standpoint. Gravel doesn’t hold water—so containers will drain better, and the plants in them will grow better, if we add a few inches of gravel to the bottom of the container.

Not so fast. Dr. Mark Rieger at the University of Georgia has utterly debunked this myth in irrefutable manner. By testing soil water density in containers of different heights, circumferences and shapes, including some into which a gravel base had been added, the good Doctor has concluded that gravel in the base of containers actually keeps the soil in the container (and plant roots) wetter than is good for most plants grown in containers.

A round, shallow container (such as a planter bowl) filled with potting soil holds more water per square inch of soil than a tall container of the same diameter. Makes sense—the soil in the tall container is stacked higher on itself, which will increase drainage, since gravity is everything.

When you add gravel to the base of a container, you in effect “shorten” the height of the soil in the container, thus decreasing the drainage. You’ll actually have better drainage (and the container or potting soil you buy will retain moisture to the exact level it was intended) if you toss a bit of screen or landscape fabric over the drainage hole, then fill the entire container with soil.

Where do I get this stuff? Well, this one is straight out of Jeff Gillman’s new book, The Truth About Garden Remedies, this month’s featured Top Pick!

"The soil under and around pine trees and oaks is acidic"

I’ve heard that for years, probably heard it before I started gardening and it isn’t true. Or rather, sometimes the soil in these areas is acidic, sometimes the soil is neutral, and sometimes the soil is alkaline.

A far greater factor to soil pH than what’s growing on top of it is what type of rock is below. Years of pine needle and oak leaf drop will lower soil pH to a mild degree, but if you’re gardening near the limestone bluffs of the Mississippi you can have centuries-old areas of oaks or pines where the soil will test 7.8 (high alkaline).

Nine times out of ten, the hosta, lamium, or ajuga you planted around the skirt of a pine, or underneath an oak, didn’t die because the soil was too acidic. The plants died because they didn’t receive A LOT of watering in their first two years to help establish a deep, healthy root system. If you relied on scant watering, or rainfall, the tree sucked it all up, and the plants underneath petered out due to dry soil, not deathly acidic soil.

Don’t automatically add lime to soil under and around oaks and pines. More often than not, you’ll be raising the soil pH from slightly acidic—which is perfect for ninety-five percent of what we grow in the north—to a fairly stout alkaline, which few plants prefer. If in doubt, get a soil test.

Related Reading: "It’s a Good Idea to Lime Your Soil"

“You always wait for the ground to freeze before applying winter mulch.”

Ah, an erroneous myth that not only stems from an erroneous myth, it adds credence to the parent myth. Still with me? Let’s try this:

Early on you learn, or should, that you apply winter mulch (if you choose to go that route) such as marsh hay, bags of leaves, etc., to your perennial beds AFTER the ground has frozen. This is often around late November/early December in Zones 3 and 4, the average date for ground freeze (frost penetrating the soil that remains until spring) being December 6 in Zone 4. You don’t mulch gardens to keep the soil from freezing, you add the insulating mulch to keep the ground frozen.

Cercis Canadensis Northern Strain

But not when you’re dealing with trees and shrubs that were planted in spring or fall and are now going to experience their first northern winter. Particularly trees such as redbuds and magnolias, and shrubs such as azaleas, rhododendrons, daphnes, quince, and more varieties of “hardy” shrub roses than the industry cares to admit. All the above are hardy to Zone 4, and in the case of shrub roses, there are varieties hardy to Zone 3.

These plants are not exactly “marginally hardy,” but you learn that getting them through the first winter is the trick. After a few seasons they hunker down for winter and emerge in spring quite nicely.

But so this doesn’t run 1,000 words, I’ll just tell you my redbud story.

I planted two redbuds (Cercis canadensis) on my humble half-acre about five years ago, both Northern Strain, developed by the University of Minnesota Landscape Arboretum. One was tree form, #10 container, I planted it near the house, the other was a clump form, BB, I planted it on the other side of my property, nearer the street.

At time of purchase they were superb specimens, well cared for by a local grower I trust. I planted them in spring, in late April, which is when redbuds (and magnolias) prefer to be planted. The soil I prepared for each was perfect, good drainage, good organic matter content, and of course I immediately mulched each with a four- to five-inch layer of shredded hardwood spread out to cover the entire planting area. I kept the soil moderately moist the first season, and they flourished.

The next spring neither budded, and by June it was clear both were stone cold dead.

I couldn’t believe it. Redbuds are allegedly hardy to Zone 4, certainly “Northern Strain,” there’s one at the Minnesota Arboretum that’s twenty-five feet tall and must be twenty years old or more. What happened?

I ran it by Gary Johnson, the venerable tree guru at the U of MN School of Horticulture, and he proposed a theory. The winter just passed—their first winter, remember—the Twin Cities received very little snow cover. We didn’t get any decent amount of snow until end of February, March. But in late January, we got our inevitable blast of Arctic air that dropped temperatures down to twenty-five below zero. Some days it was a bit warmer than that, but basically the bitter cold hung in for about three weeks.

Without snow cover (insulation), Gary theorized that the frost penetrated the soil so deeply that it got in around the three-foot diameter circle of hardwood mulch surrounding the trees, then crept underneath, basically freeze-drying the entire root clump of each tree. Had I extended the shredded bark mulch wider around each tree, it still wouldn’t have helped.

Realizing that in this case, the shredded hardwood mulch I had laid down in spring wasn’t enough of a winter mulch to keep frost from penetrating so severely without a helping hand from snow cover, I asked, why not insulate around first- and second-year, dicey plantings with not only shredded hardwood, but a ten-inch layer of marsh hay, applied in early November, before the ground freezes, and applied in a much larger circle around the trees, to keep the frost from penetrating so deeply? Probably would help, said Gary.

November mulching with marsh hay

So I dug up my $350.00 worth of copper sculptures, planted two more redbuds, and that fall, that’s exactly what I did. The following spring (and each year thereafter), they jumped to life as soon as the spring sun warmed the soil (I removed the marsh hay in early April).

Now, that’s what I do for many of my “first winter” plantings. In early November, I mulch newly planted shrubs and trees (from the list above) with marsh hay, going out six feet from the plant, if there’s room. And I fluff the hay right up around the trunk, as you would not do with a summer mulch, because the tree is going dormant, and a ten-inch layer of fluffed marsh hay is not going to make it think you’ve raised the soil level.

And the winterkill and winter dieback rate has plummeted. Particularly for shrub roses, that, left to face winter with just their hardwood mulch, may not die, but often, that first spring, experience a fifty-percent or more dieback from winter.

It’s not a bad idea to use this trick in the north for any newly planted evergreen, particularly low growing varieties of junipers, arborivitae, and yews. Mound marsh hay on top until you can’t see the plant. If we receive scant snow cover during winter, winterburn will not occur.

So I’m mulching in these cases not to keep the ground frozen, but to keep it from freezing so severely.

There—921 words.