How To Pick, Plant And Propagate Perennial Plants In Gardens

University Place: How Sun, Shade, Soil And Other Factors Figure In Their Growth
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Dan Mullen (CC BY-NC-ND 2.0)
Johnson: "Daylilies are one of the backbones of the perennial garden."

Gardeners around Wisconsin have thousands of choices when it comes to cultivating perennials, but success starts before they ever bring plants home from the nursery.

When shopping for perennials, it's important to first ask: "Is it going to survive the darned winter?" Lisa Johnson, horticulture educator for the University of Wisconsin-Extension Dane County, posed this question in a June 26, 2015 talk recorded for Wisconsin Public Television's University Place. A perennial's winter hardiness starts with its tolerance for the cold, she said, but it doesn't end there.

The U.S. Department of Agriculture's Plant Hardiness Zone Map is the standard by which gardeners can determine which plants are likely to thrive where they live. The zones are determined by an average of the 5-degree range of typical low temperatures for a given location. The lower the number, the lower the temperature a plant can withstand and bloom again in the spring. Plant tags typically include information about their hardiness zone.

Many other factors figure into the life of a perennial, though. For example, snow cover helps to insulate plants, Johnson said. The duration or severity of a cold snap can also affect a perennial's chance of overwintering, as can wild temperature swings in fall as plants are preparing to go dormant for the winter season. Soil type plays a role in a plant's winter hardiness as well, she added.

Because most perennials only bloom for a few weeks each year, it's important to choose plants with interesting foliage, Johnson suggested. She likes to "mix it up" and plant perennials with bold, broad leaves next to those with lacy leaves, or pair tall, grassy varieties with those that grow in clumps.

"I'm not advocating that every single plant in your garden should be purple or orange or pink or have really colorful foliage like that, but sprinkling a few of these plants in amongst the green really does add a lot of excitement and visual interest," Johnson said.

Planning —and planting — a perennial garden requires some sun-watching as well. A full-sun area is one that gets six or more hours of full, uninterrupted sun, Johnson said, but afternoon sunshine is more intense than morning sun.

Shade is a little harder to define, she said.

"There are always multiple degrees of shade. People will say, 'Well, what is full shade versus part shade?' In general, when I'm talking about shade I'm looking at shade that is cast by buildings or under trees."

But, Johnson added, "If turf doesn't grow there, you've got a pretty good clue that you've got full shade."

Key facts:

  • Full sun consists of six or more hours of uninterrupted sun. Afternoon sun, from the west, is more intense than morning sun, from the east. Areas underneath trees that get dappled light are considered partial shade. Full-shade areas are those underneath dense trees, such as Norway pines, or in areas that get very little sunlight. A light-colored house will reflect some light on fully shaded areas.

  • Plants located in full sun areas but spaced closely together may behave as if they are not getting full sun.

  • For proper planting, roots at the bottom third or quarter of a plant should be gently loosened after it's pulled from a planter. If it's sold in a peat pot, that container should be removed before planting as well. It's important not to bury the roots, as well as to surround the plant — without smothering it — with a maximum of two to three inches of mulch.

  • Gardeners should beware plants that Johnson calls "garden thugs," pointing to words like "vigorous" or "spreads" on their tags. Such plants may also have a lot of roots protruding from the bottom of its pot. These are good clues that it may not play well with others and that gardeners will need to keep an eye on it if planting it.

  • Watering by hand is preferable to using a sprinkler. Plants should be watered at their base, with the amount adjusted depending on whether the plant prefers moist or drier soil. With the exception of succulents, drip irrigation is also a good watering method for perennials.

  • New plants are best monitored carefully to see how much water each one needs. Depending on weather conditions, new plants may need water two to three times per week until they are established. Each plant should be watered for a count of six. It's also important to make sure the water is getting through the mulch layer into the soil.

  • Plants benefit from being propagated, and most are propagated by division. Though many plants can be divided any time of year, it is generally best to do it in spring. It's best to avoid dividing plants while they are flowering, or during hot, dry weather. Plants also shouldn't be divided in late September or October because they won't have time to re-establish themselves before the ground freezes. (Peonies are the exception to this rule.) A sharpened shovel or butcher knife can be used for dividing plants. Plants with rhizomes (underground stems) or taproots, such as Russian Sage, dianthus, coreopsis or salvia, are more difficult to divide.

Key quotes:

  • On which perennials are good selections: "I like to choose plants that are somewhat low maintenance and have a long blooming period to give me 'the biggest bang for my buck.' I don't even buy plants that are not disease resistant."

  • On the importance of soil types for perennial gardeners: "A lot of us who live in urban areas or suburban areas have lots of little microclimates. There are areas in my yard, for example, where I have heavy clay, then there's another area where for some reason it's more sandy, and then there's areas of the yard that have acceptable clay-loam soil. My rule of thumb is that I don't give up on them until I've killed them in three different places. If it doesn't work in one soil area, I try it in another one. If I've tried it in all three areas, I figure it's a winter hardiness issue and not a soil issue."

  • On the difference between sunshine and heat to perennials: "Sun and heat are not necessarily the same thing. Some plants that are adapted to full-sun areas may still not like being planted on the south side of a white house where they get reflected heat and light off that building, or dry and hot areas. So it's important to do your research … before you put a plant in a stressful situation."

  • On the timing of dividing plants: "How do you tell when it’s time to divide a plant? When the center begins to die out and it resembles a doughnut. That's the plant saying 'It's time to propagate me!'"

  • On the importance of the daylily: "Daylilies are one of the backbones of the perennial garden. They grow in a nice clump. They bloom pretty much in the middle of the season. You can grow them all the way up into Zone 3, so if you have enough sun for them — they should perform well. There's been a lot of breeding done with daylilies, too. There’s something like 30,000 cultivars of daylilies, so if you pick up a catalog from a daylily company and you look at another daylily company's catalog, there's often very little overlap."
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