Ask the Master Gardener: Tricks of the trade for container gardening

The first key to success is using a good potting soil.

A Peony "Do Tell." Contributed / Jennifer Knutson

Dear Master Gardener: My containers look a little sparse. How many plants should I put in a pot? Are there any “tricks of the trade” for planting and designing containers?

Answer: Container gardening can be a rewarding way to grow plants and add interest to your landscape. The key to success is a good potting soil. It will strengthen the roots of the plants by allowing good air circulation and holding moisture without staying too wet. Look for brands of potting soil that contain Canadian sphagnum peat moss, coir, ground bark, and perlite. If yours doesn’t contain a small amount of fertilizer, add a granulated slow-release fertilizer when you plant. Don’t use regular garden soil because it is too heavy, compacts too easily, and drains poorly.

The classic design formula is to have a thriller (focal point plant), fillers (to provide color and texture), and spillers (trailing plants). Place the tallest plant (thriller) in the back center or center with smaller plants around the base of the tall plant (fillers), and trailing plants at the container’s edge to drape over the side (spillers). Determine your color scheme. If you like a more formal look, choose one color; for a less formal look choose several colors. Select plants that grow well together and have the same water and light requirements. Check tags for light requirements: full sun means six or more hours of sunlight, part sun/part shade is 3-6 hours of sunlight (preferably in the morning), and full shade is less than three hours of direct sunlight

How many plants? Larger plants provide immediate impact, but cost more. Smaller plants cost less, but need more time to fill in. Placing as many plants as possible into a container will make an immediate impression. However, when plants are crowded together, they are more prone to disease problems. If you allow enough room between plants so the container will look full after two to three weeks of growth, the plants will be healthier. They will have better root growth and more air circulation, decreasing the potential for disease. To give you an example - in my 12-inch square containers that are in the sun and accessible to deer, I plant a spike in the middle for the thriller, four marigolds from cell packs for the fillers, and two nasturtiums from cell packs for the spillers. At first the containers seem a little sparse, but two weeks later they are filled in. I hesitate to say this for fear the deer will now prove me wrong, but they do not seem to like the taste of these plants and I have had good luck for years. I have also used sweet alyssum or lobelia successfully as a spiller in the past. Cleome has an odor and thorns to make it unpalatable to deer and it makes a great thriller in a container. Cannas seem to be another “safe” thriller. Other annuals that are reported to be “deer-resistant” are: marigolds, zinnias, sweet alyssum, lobelia, heliotrope, ageratum, nasturtium, calendula, spikes, and Persian shield. In containers that are protected from deer you can get more creative.

To keep containers looking good all summer, fertilize with a liquid fertilizer every two weeks. Water when the soil feels dry, usually daily. Deadhead those annuals that are not self-cleaning.


Dear Master Gardener: My roses have aphids. Should I spray them with something?

Answer: The main insect pest of roses is the aphid. Aphids do some aesthetic damage, but do not spread disease or kill the rose bush. Aphids tend to stick together on a plant making them an easy target for predator insects. Many of the natural enemies of aphids are more susceptible to chemical control than the aphids, so the only liquid you should spray on them is water. Knock the aphids off your roses with sprays of water from your hose. They won’t climb back up.

Dear Master Gardener: I bought a peony last year, but it doesn’t have very many buds. Should I fertilize it? Also, two peonies flop over when it rains. Should I stake them?

Answer: Hopefully, your new peony was planted in well-drained soil in full sun. Don’t be alarmed if your one-year old peony doesn’t have many buds or flowers this year. In fact, it’s best to remove all but one or two of the buds so the plant will put its energy into developing strong roots. It often takes two to three years for peonies to become well-established and bloom. The most common cause of peonies not blooming well is they were planted too deeply. They should be planted with the eyes 2 inches below the soil surface. Peonies do not need much fertilizing, if any. Too much nitrogen will cause the plant to produce a lot of foliage and reduce the number of blooms. If you do fertilize, do it right after they are done blooming and either use a balanced fertilizer or one with a higher phosphorous content (middle number on fertilizer package).

Some peonies have stronger stems than others and don’t need support. Peony hoops are helpful for heirloom varieties, which tend to have weaker stems and huge double flowers that cause the stems to flop over when they’ve been weighed down by rain. The most successful supports are usually the double ring hoops. Install them in early spring before the plant gets too large and it becomes a two-person job. As clumps mature and get larger, they may not fit. Unfortunately, the hoops are not without problems, as stems bend where they contact the upper metal ring during strong winds and/or rain. If my heirloom varieties flop over the peony hoops from a storm, I end up with gorgeous bouquets in my house. They make a beautiful, long-lasting cut flower.

Dear Master Gardener: My asparagus have beetles on them. They are oval-shaped, about one-fourth inch long, bluish-black in color with six cream-colored spots on their backs. What should I do?

Answer: There are two kinds of beetles that attack asparagus and the one you have described sounds like the common asparagus beetle. It is the more common of the two and unfortunately causes more damage. The beetles feed on asparagus spears, causing browning and scarring. Be on the lookout for these pests starting in May just after the plants emerge and continue monitoring them for the rest of the growing season. The best time to check for asparagus beetles is in the afternoon when they are the most active. You can manage common asparagus beetles by handpicking them and dropping them into a bucket of soapy water.

You may get your garden questions answered by calling the new Master Gardener Help Line at 218-824-1068 and leaving a message. A Master Gardener will return your call. Or, emailing me at and I will answer you in the column if space allows.
University of Minnesota Extension Master Gardeners are trained and certified volunteers for the University of Minnesota Extension. Information given in this column is based on university research.


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