CONTAINER GARDENING 101 AND GARDENING ZONES

Someone (one of you!) sent me an email recently about picking out plants for a container and we got to chatting about the basics of container gardening and also gardening hardiness zones. Since the information seemed to be helpful to a beginner, I figured I’d whip up a post about it for anyone else out there just getting started! This will be very high-level information to help get you familiar with both of these subjects.

Hardiness Zones

If you’ve shopped at a garden center for outdoor plants, you’ve probably noticed a zone mentioned on the tag. Something like “zones 5-9” or “hardy down to zone 6”. Those zones were put together by the USDA using the minimum winter temperature that occurs on average in that area. Zone 1 is the coldest (think northern states or high altitudes) and zone 13 is the highest (think southern US, Hawaii, Puerto Rico). Also, coastal areas are generally in higher zones than their inland counterparts in the same state. For example: my friend lives in Seattle Zone 8 but most of Washington State is in zone 6. Here in Pittsburgh, we’re in zone 6. That means our average annual extreme low temperature is -10 degrees F. Zone 7’s is 0 degrees. Zone 8 is +10 degrees, and so on. They also subdivide each zone into “a” and “b” but that’s not really too important.

Here’s a lovely link to a map of the zones in the US. Also, this link allows you to put in your zip code to find your zone and to zoom into each state to see the lay of the land. Here’s a slightly blurry version of it too:

Hardiness zones are also how they determine if something is an annual or perennial in your area. An annual for us here in Pittsburgh is usually a more tropical plant (say a Petunia that’s hardy down to zone 10) that we plop in the ground in May, get a great showing of blooms all summer long, and pull it out when the frost comes in October. That annual is actually a perennial in those warmer zones, but for us, we can rely on just one season of beauty.

Our perennials, shrubs, and trees here in Pittsburgh that come back year after year are plants that are hardy in zone 6. So those may be labeled as “zone 4-8” or “perennial in zone 6” or something like that. As long as zone 6 is within the range, you’re good to plant it in-ground and have it comes back next year (assuming you take good care of it). If they’re hardy down to zone 5 or 4 then that’s even better because they can most likely survive a tough winter or a freak polar vortex that passes through.

A fun fact about zones ~that is probably more Zones 201 content~ is that some plants, like tulips, need a cold period for them to grow properly. In Pittsburgh, we have the cold temps they need, that’s why we can plant tulip bulbs in the fall and they’ll come up in the spring to bloom and be eaten by deer. However, people in warmer zones (like in Raleigh, NC zone 7) have to put their tulip bulbs in the fridge for several weeks over the winter to mimic that cold period and then plant them out in the spring for spring blooms. Crazy.

So those are some general details about zones. Let’s move on to containers and then I’ll explain why these two topics are related.

Container Gardening

Planting in containers is pretty much the same as planting in-ground but there are some design elements that I’ve heard mentioned over the years that I think help beginners put together a nice container. I did not come up with this and I don’t know who did but I think by now it’s just a general gardening rule-of-thumb (that you’re welcome to break).

The three elements that make up a full, lush container are thrillers, fillers, and spillers.

Thrillers will be a large plant(s) that will be the height, the drama, the centerpiece. Look at the heights on plant tags while shopping and grab something that will add a tall element to your container. Some easy options would be a purple fountain grass that gets multiple feet tall or salvia that will get a foot or two tall.

Fillers will be smaller plants that fill in around the base of the thriller plant. Usually, there are a few of these planted around the edge of the container to fill it all in. Feel free to mix and match varieties and bloom colors, like different colored mounding petunias or other plants that stay roughly under a foot tall. (Fillers are usually the one element I’ll skip in order to save a couple of bucks - plants get to be expensive, man!)

Spillers will be vining plants that drape over the edge of the container. You could try an ivy, trailing flowers, or a sweet potato vine. You can tuck a few of these in between the fillers and they’ll grow over the edge.

You can use this framework for containers that you’ll see from all angles or for containers up against a wall or other structure. You can just adjust plant placement to accommodate. For containers you’ll see from all angles, you can use your thriller as the centerpiece and then plant your fillers and spillers on the perimeter of the container. For containers up against a wall, you can place your thriller at the back of the container, the fillers in the middle, and the spillers in the front.

Try to choose plants that require the same water and light requirements. If your planter will be in sun all day, get plants that are labeled for full sun. Will your container be under a shady porch? Grab shade-loving plants. The plant tags will also kind of help you out with watering requirements, but not really. I think it’s safe to say that most outdoor plants need to be watered (or rained on) a couple of times per week during our hot, humid Pittsburgh summers. However, some plants are labeled “drought-tolerant” and can be watered less, so make sure you’re aware of that when planning out your containers.

Two other things to note when using perennials, shrubs, and trees in containers: 1. It probably won’t survive in the container forever. After a few years, it may become root-bound and will want to be planted out into the landscape or at least into a larger container. 2. Remember to water it every couple of weeks over the winter, especially if it is not getting any rain or snowmelt. It’s still alive and needs water even in the colder months!

Now, on to why zones are important in container gardening. You can fill up your containers each year with new annuals, enjoy them for the season, and then start over the next season, or you can use shrubs, trees, or perennials that will either come back year after year or provide yearlong interest, especially if you choose evergreens (maybe like a boxwood or evergreen topiary). When using perennials, shrubs, or trees in containers, try to find varieties that are one to two zones lower than your gardening zone, which would be plants hardy down to zone 4 or 5 here in Pittsburgh. The reason for this is that when you’re keeping plants in containers over the winter, they don’t have much protecting their roots from the cold temperatures - just a few inches of soil and the container. Compared to plants that are planted in the ground that have soil and grass and mulch and other forms of protection, that’s not that much. So, get a plant hardy to a colder zone and you’ll have more of a chance of it surviving through the winter.

My front containers last year with a perennial gaura (which I promptly killed) and a powerhouse-of-a-plant: the Supertunia Bubblegum which put out hot pink blooms all season long. This is an example where I skipped the filler and just used a thriller and spiller.