All about Gardening in Containers

To be honest, container gardens are more work than in-ground gardens, the exception being potted succulents, like my sedum collection that happily bakes in the sun with no supplemental watering.  But sometimes we grow plants despite the trouble they cause us because we love them, or maybe because balcony or rooftop gardening is our only option.

Containers are also great when our soil is horrible for planting. And containers are portable, so we can move plants around the garden or even indoors for the winter, especially with the help of plant dollies.

The Container

Types are detailed below but there’s one thing they all have in common: the bigger, the better for the health of the plants. Sources recommend containers that are as wide as they are tall for stability and for good insulation of roots.

  • Plastic pots are inexpensive and are slow to dry out, which is an advantage with most plants, but a disadvantage for plants that need “good drainage” — like lavender or these sedums. And definitely on the negative side, they look cheap.
  • Terra cotta pots look better, but are a lot heavier, and most won’t withstand the freeze-and-thaw cycles of winter. They clearly provide better drainage than plastic, though.
  • Glazed ceramic pots look great but need more drainage holes, and again if you live where winters include freezing temps, make sure they’re hardy before you buy it, unless you’ll be bringing them indoors.
  • Cheap wood can and will rot; redwood and cedar will last longer. There are concerns about wooden containers that have been treated — fear that the vapors can damage plants, and here’s some information about treated wood and plants.
  • These days a very popular alternative are the lightweight fiberglass and resin planters that mimic real stone, terra cotta, or concrete. In the photo above, most of the planters are fiberglass and they cost about $60 each.
  • Hanging baskets are often lined with sphagnum moss to better hold water. Still, it’s hard to water often enough if the basket is hung in the hot afternoon sun. Hanging plants require a special commitment on the part of their caretaker.

Landscape Designer Michelle Derviss suggests choosing containers that match the color or style of the house.

Design

On GardenRant Elizabeth asked the design question: To mix plants within one pot or not? and got some great responses. The debate is between plantings of mixed plants or filling each pot with lots of only one particular plant.

Ginny Stibolt suggests choosing plants for three levels: “the tall or spiky plant, the bushy medium height plant, and the vining or trailing plant that will hang gracefully from the sides. Work with odd numbers of plants, and maybe add a nice rock or art object to enhance the arrangement. You can accomplish these arrangements in one large, stand-alone container or by using several containers of different sorts with a variety of plants. Mostly I’ve had better luck with single species pots.” This three-layer idea is practically universal (certainly in Japanese flower arranging) and Fine Gardening Magazine coined the terms “Thrillers, Spillers and Fillers” for them.

Don Engebretson, the Renegade Gardener, writes: “Plants grown in pots and other containers open up a whole new dimension of design possibilities. One of the best gardeners I know plants coleus in12″-tall pots in the spring, then places the pots strategically throughout one of her numerous flower beds. By July the perennials and annuals rising from the earth completely hide the pots; all one sees are these great plumes of coleus bursting up 18″ to 20″ above the ground and one thinks, wow, can she ever grow coleus!”

Don recommends just one pot of flowers or cluster of pots to one side of the front door rather than pots evenly matched on both sides. And he likes oddness: “The odd-numbered arrangement will always look better than two or four or six pots together. No one knows why.”

The Potting Medium

These days, soil-less potting “media” are popular — all bagged up at the hardware store or garden center. But Ginny thinks they’re fine for annuals but for potted-up plants that will be staying in place longer than a year she recommends “adding some compost in the soil mix and as an occasional topdressing because all those good microbes create a better environment for the roots.” She uses half compost and half sandy soil for her pots, with a sterile potting medium only for the top two inches and mulch on top of that to prevent weeds. Sounds good to me.

Those soil-less mixtures have the advantage of being lightweight, well-draining and disease-free, but DO need all their nutrients provided by you, don’t forget. In any event, don’t use regular garden soil — it’s heavy, not as freely draining (which is of supreme importance in containers) and it may come with pests.

Now what about using those “water-saving crystals” or hydrogels we see in the stores? Well, garden writers discussed this at length via email and one of them summed it up nicely here on her blog. The bottom line? “Just say no” to them.

What about covering the drainage hole?

It’s practically gardening tradition to cover drainage holes with broken terra cotta chards or an inch of gravel, but respected horticultural authority Linda Chalker-Scott says studies have shown that the practice actually impedes drainage - because water does not travel well from the potting mix to the coarse gravel mixture. Plus, plant roots need all the soil they can get.

To prevent soil from washing out of the drainage holes, place a piece of weed barrier cloth, or even a few dried leaves or several layers of newspaper over the holes before adding the soil. Ginny again suggests using “a large piece of the non-woven weed barrier cloth so it extends up the sides of the pot, it can also discourage ants from using your outdoor pots for nests.”

Planting

Fill the container with the growing medium to within one inch from the rim — to leave room for watering without water spilling over the side. Then give it a good soaking before planting. Insert plants so they meet the soil again about an inch from the pot’s rim and add or remove soil so that the soil surface is smooth and also an inch below the rim. Then add a half-inch of mulch to preserve moisture, prevent weeds and most obviously, to keep soil from splashing up onto your plants — ugh. What to use? Pebbles can bake the plants, which is fine for succulents. Otherwise use a mulch that cools, like bark chips or my favorite, shredded pine bark.

Watering

If they’re sitting in the midday or afternoon sun, most pots need watering daily, if not more often. Automatic watering systems can work like a dream, I’m told. I recently saw one that snaked up the posts of a deck to automatically water all the pots on the deck. I could use one of those myself.

Feeding

Some experts recommend including fertilizer with every other watering, which seems like a lot. I agree with the other experts who suggest every two or three weeks. It’s best to use an organic fertilizer that’s diluted in water, like fish emulsion and liquid seaweed.

Choosing plants

As a general rule always pick plants that are hardier to colder winters than where you live because soil that’s aboveground gets colder. So, for example, in my Zone 7a garden I choose plants hardy to Zone 5 or colder for pot. (Most of my in-ground plants are a zone or two hardier than technically required, too, but when plants are in pots it’s mandatory.) Then take note of a plant’s watering requirements and ask yourself if you’ll really be watchful about watering daily, for instance. Next, how much sun ya got?

My favorite perennial for pots in the sun you already know — sedums of all types. I also love grasses with trailing plants, the photo right being a stunning example of combination.

The best-looking shrub-in-a-pot I’ve grown is that most malleable of shrubs, the acuba. More recommendations coming here soon.

I’ve tried some trees in pots, Korean and Bosnian pines, and a couple of short junipers, but one of the pines quickly outgrew its pot and the other one I feared wouldn’t last long, so I’ve moved them both to the garden. Why wouldn’t they last long, you ask? When their roots expand outward enough to contact the pot itself, those freeze-and-thaw cycles are murder on them. Paul James confirmed the short-lived nature of most trees in pots recently, as did a nurseryman specializing in conifers, when questioned by me at a trade show. One possible solution I’ve seen suggested is to line the inside of the pot with a thin layer of packing wrap, which will expand and contract against the side of the pot — or at least that’s the idea. (See more about wintering over below.)

Got a deck you’d like to enliven with flowering vines? Containers are the way to go, there being no open ground on your deck, and here’s what the National Arboretum’s Scott Aker suggests:

Trumpet vine (Campus radicans), because it attracts hummingbirds and is native to this region, though he considers the Chinese version, Campsis grandiflora, even nicer. C. grandiflora ‘Morning Calm’ is commonly available and is compact, with larger flowers than most trumpet vines. Its flowers are a mix of peach, pastel orange and light yellow.

He also recommends large containers — a cubic yard of potting soil to support enough vine to at least partially cover a pergola, for example. If a container that size looks bad on the deck, maybe it can be put on the ground (as in this photo) and the vines trained up and over the structure you’re trying to cover.

And asked about clematis, Aker warns that they really “resent” hot weather and he’s seen them grown successfully rarely, and only when given some protection from the sun.

Plants for Shade or Partial Sun

I have lots of plant suggestions here, thanks to Joel Lerner in the Washington Post. For something you can leave outdoors all year he suggests trees, either deciduous or evergreen, plus some perennials or annuals for color — and all but the annuals will winter over.

For woody plants, he suggests: Japanese maple, Kousa dogwood, ‘Little Henry’ Virginia sweetspire, ‘Emerald’ arborvitae, ‘Hetz Midget’ arborvitae, dwarf falsecypress ‘Nana Gracilis,’ golden variegated euonymus (E. japonicus ‘Aureomarginatus’) and wintergreen boxwood.

Lerner suggests these perennials for pots in the shade : Rhodea, hellebore, astilbe, Japanese painted fern, evergreen wood fern, bleeding heart ‘Luxuriant’, hosta, columbine, cardinal flower, hardy begonia, and woodland phlox.

And his favorite annuals for pots are: coleus, impatiens, Joseph’s coat, caladium, peacock ginger, elephant ear, rex and tuberous begonias, and bacopa.

What about Winter?

First, will your plants survive winter? (I’ll repeat here: they’d better be hardy to at least one zone colder than where you live.) Many gardeners bring their pots indoors for the winter, preferably to a basement or garage that stays between 40 and 60 degrees F. (But who has that? My garage gets too cold and my basement stays too warm.) Root cellars may be best but again, who has that?) If potted plants are tropical they can usually be brought indoors to winter over as houseplants, assuming you cut way back on the watering and give them the right type and amount of sun.

Or you can do what’s frequently suggested — leave your not-hardy-enough plants outdoors but wrap them up in burlap or bubble wrap or something equally unattractive. I find it easy to say no thanks to that idea.

And check this site for some great-looking winter container ideas.

Photo and design credit: Michelle Derviss.

 

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