Container gardening has traditionally been focused on the use of annual plants. But containers are a great way to garden, and we can also be using them to grow many perennials and perennial ornamental grasses. In fact, gardeners living east of the Mississippi will find the use of containers as a great way to expand their palette of growable perennials, especially xeric ones that would be unhappy in the ground because of excessive moisture and soggy freeze-thaw conditions in spring. Perennials that may seem impossible to grow in the ground become easily cultivated in pots.
A Few Container Gardening Basics
Use bigger pots. (Note: I use the words container and pot interchangeably.) Avoid using pots that are too small. Plants quickly become root-bound and it becomes difficult if not impossible to keep them adequately watered and fertilized. I always use at least a 14 inch diameter pot. Remember: The bigger the mature plant, the bigger the pot.
Always use a soil-less potting mix. Filling pots with garden soil is a recipe for failure. Garden soil compacts and greatly restricts drainage and air exchange. I always recommend a high quality soil-less potting mix. This potting mix can be reused each season and enhanced with new ingredients.
Keep the plants well fertilized. Because we must water pots more frequently than plants in the ground, we need to replenish nutrients that are flushed away. If you want to grow your pots organically, top-dress every couple of weeks with earthworm compost and Yum Yum Mix. Compost tea is also excellent. Growing conventionally, use Osmocote slow release fertilizer mixed into the soil at potting time and supplement with water-soluble Miracle-Gro (or equivalent) once every week or two.
Don't put gravel in the bottom of the pot; this is a useless technique that can actually restrict drainage in the pots. Fill the pots completely with soil-less mix. If the pot has a large drainage hole, I'll put an irregular rock over the hole that doesn't seal the hole and allows for water to flow out.
Leaving pots out-of-doors year-round: If you want to leave your containers outside year-round, I recommend using a fiberglass pot or the pot-in-pot strategy to avoid cracked pots and cold damaged roots. For pot-in-pot cultivation, plant in a plastic nursery container and drop this pot into a slightly larger ceramic pot. Fill the empty space in between with small bark nuggets. This insulates the inside pot from heat and cold and allows winter watering without cracking the ceramic pot.
Protecting Pots Over the Winter
We need to protect the pots and their resident perennial plants from the extremes of the winter weather.
After several hard frosts, move them into an unheated garage or cold frame.
If this isn't practical or you have a lot of pots to protect, make a straw bale enclosure where the pots can be placed. Cover with a couple of sturdy pieces of row crop cover (frost blanket) fabric.
Be sure to give the pots a drink every month or so on a warm day so the roots don't dry out excessively.
In spring, cut the perennials back and move them into the outside position.
When using perennial plants, they can remain in the pot for at least two seasons before re-potting them into a larger one. Or the perennials can be divided and re-planted back into the same pot with fresh soil-less mix.
When designing your container garden, make sure pots have a mix of plants: thrillers, fillers and spillers. Perennial plants that appreciate well-drained soil are a great choice for containers.
Arranging Plants in the Container
Remember the container gardening mantra, "thrillers, fillers, and spillers." The tallest, showiest plants (thrillers) go in the middle of the pot. The medium-sized plants (fillers) go around the center plant. Soften the edges of the pots by planting trailing (spillers). If you're going to place a pot a container (pot) against a wall, put the tallest plants on the wall-side of the pot.