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Overwintering Marginal Plants

I keep seeing photos of interesting plants I’d like to grow, but they’re labeled zone 6 and I’m in zone 5. What can I do to successfully overwinter these marginal plants? I’d like to try them, but I don’t want to waste my money.

“Pushing a plant past the limits of its hardiness is not for the average gardener,” says Marie Dvorak of The Planter’s Palette in Winfield, “but plants can sometimes be successfully overwintered by siting them in a special microclimate such as a sheltered southern exposure.” Another tip comes from horticulturist Ann Hancock at Michigan State University, where they are trialing perennials that the English firm Blooms of Bressingham is introducing in the U.S. Hancock explains that the best way to keep perennials that are tender in Zone 5 or that are rated for Zone 6 is to grow them in raised beds with good drainage because poorly drained wet soils will cause the roots to rot. In addition, it is important to check perennial beds for plants that have heaved out of the soil through repeated freezing and thawing. Replace them in the soil and protect with mulch.

One example of a marginally hardy plant that will be getting attention this year is ‘Flamenco’, a new cultivar of Tritoma (Kniphofia, red-hot poker) that has just been named an All-America Selections Winner for 1999. Tritomas should have their foliage tied upright so water does not remain in the crown, said Hancock.


Fertilizer

What ratio and amounts of fertilizer would you use for a perennial bed and a vegetable garden? For growing annuals in a greenhouse, should the fertilizer be fast or slow-release, organic or inorganic?

Use 3 to 5 pounds of a 10-5-10 or 10-10-10 fertilizer per 1000 square feet in the perennial bed. Add leaves as mulch. In your vegetable bed, nutrients are important. Fertilize more heavily using a 10-10-10 fertilizer at the rate of 5 to 10 pounds per 1000 square feet.

For hanging baskets of annuals in a greenhouse, a slow-release fertilizer is best. Use a 20-10-20 liquid fertilizer following label directions. In general, organic fertilizers have a lower chemical analysis than inorganic ones, so you have to use more, according to David Tyznik, owner of Planter’s Palette, Winfield.

A healthy plant is dependent on its soil. A balanced soil is rich in beneficial microorganisms that improve the soil structure and supply necessary nutrients. Soilless potting mixes have no microorganisms. “Both organic and inorganic fertilizers work well, but those in soilless mixes need more fertilizer,” says Tyznik.


Hibiscus

I am growing my potted tropical hibiscus indoors for the winter. The leaves are starting to yellow and fall off. Should I give the plant iron and should I fertilize it? Do I cut it back, and if so, when?

Hibiscus leaves turn yellow in response to the change in environment. The older leaves fall off first. If the leaves are chronically pale yellow with green veins, the plant could be lacking iron, but in your case, it is probably the change in growing conditions that is affecting the plant. When the days get longer in February or March, prune it to shape it, or cut it down to force good bottom growth.”

“Don’t panic; the plant expresses itself dramatically,” reassures Mike Geimer, owner of Geimer Greenhouses, Long Grove. “Don’t be timid pruning it.”


Staking Plants

I dislike staking perennials. Is there anything I can do to avoid it?

There are several techniques you can use to limit time spent on this chore, but it is difficult to totally

avoid staking some plants.

First and foremost, the old adage “right plant, right place” means full sun perennials need to be grown in full sun for tight upright growth. If they are too shaded, the plant will stretch or bend toward the sun and grow in a loose habit. Growing plants a little closer together also helps because it enables plants to support each other.

The best remedy to avoid staking is to cut some tall plants down by one-half in June. That will delay flowering in helenium, monarda, platycodon, chrysanthemum and aster, but the plants will remain upright. If you really want to eliminate cutting back, grow dwarf versions of these plants.”
Short, thin branches from forsythia or dogwood (known as “pea staking” in England) can be placed around the perimeter of newly emerging plants to help keep the stems upright. Some garden centers have begun selling bundles of branches for this purpose. In a short time, the shrub cuttings are covered by emerging shoots. This technique works well with fine foliage such as baby’s breath or flax.

There are numerous staking supports from bamboo to iron rebar rods. Wire staking supports are available in full, half and even quarter circles to support single stems. Peony hoops can be left in place over winter and will support not only peonies but boltonia, baptisia, and even errant tomatoes.

Some plants always need to be staked. Giant dahlia tubers need a heavy stake dug into the hole at planting time. That prized, single-stemmed iris, lily, or delphinium can get blasted in a storm, and then it’s too late to save it.

Remember: the time to stake plants is during the cooler days of spring when plants are growing rapidly and not on humid July days. Staking should be done unobtrusively and subtly for plants to retain their natural grace.


Are there any shrubs that can survive in exposed situations?

At the end of every winter, there are many shrubs growing along sidewalks that are dead and damaged either by salt, wind or dogs. Are there any shrubs that I can plant in these more exposed situations and expect them to survive?

Sharon Yiesla, plant knowledge specialist, The Morton Arboretum, Lisle

First, we need to note that there are some exposed areas where we just shouldn’t try to grow plants. There’s too much ice melt or salt, and dog urine is always a problem. Evergreens are also out. There’s nothing that will tolerate wind or salt. That said, there are some deciduous shrubs that can work. Hydrangea are now a huge group, Potentilla tolerates everything, and fragrant sumac (Rhus aromatica) can survive shopping center parking lots. Rose of Sharon (Hibiscus syriacus) is tough, but look for double-flowered cultivars so they don’t go to seed so fast.


Peter Moersch, owner, Stonewall Nursery, Oregon, Wisc.

I would advise looking for something on a standard. A standard is a genetically compatible rootstock which has had another plant grafted onto it. Standards are usually 24–48 inches tall and serve to “pick up” the grafted section off the ground and make it more visible. This certainly helps with snow load and keeping plants away from doggies.

Many dwarf conifers and deciduous materials are used as grafted standards. Some examples would be dwarf white pine (Pinus strobus ‘Horsford’), globular blue spruce (Picea pungens ‘St. Mary’s Broom’), and Siberian cypress (Microbiota decussata). Many dwarf Korean lilacs (Syringa spp.) and hydrangeas are also done on standards.


Scott Stringer, certified arborist, Bartlett Tree Experts, Chicago

Starting your planting project with a soil test can ensure your tree or shrub selection has the best chance of survival and can tolerate existing salt levels. Options for shrub planting with a tolerance to salt are Hydrangea, witch hazel (Hamamelis virginiana), Spiraea, serviceberry (Amelanchier canadensis), and rose of Sharon (Hibiscus syriacus). Options for trees are Japanese tree lilac (Syringa amurensis), swamp white oak (Quercus bicolor), and in wet areas, bald cypress (Taxodium distichum).


Osteospermum

Can you tell me if the African daisy Osteospermum ‘Springstar Aurora’ can be winterized here? It is a healthy plant?

Those South African daisies could never endure a frigid winter here and should be considered annuals. Their daisy-like flowers with blue centers open only in sunlight and bloom best in early summer or later when summer days cool down. They can be seed-grown under lights.


Sowing Seeds

I would like to start seeds under lights. When is the best time to start flower seeds? The seed packet always says to sow a number of weeks before the last frost. When is the last frost?

“In this area we use May 15 as a benchmark for the last frost date, but it can be earlier or later,” says Chuck Prosek, grower at Prosek’s Greenhouse, Winfield.

If your seed packet gives you the number of days to germination, add that to the number of weeks for growth, and then count the number of weeks back from May 15. Tomatoes sown indoors generally need 6-8 weeks of growth before being set out.

Many seed packets do not include germination time. A good guess is to allow 1-2 weeks for the seeds to germinate, but if you use bottom heat, your seeds will germinate earlier and grow faster. Most seeds need warm soil of 65 to 70 degrees to germinate and grow.

When you sow seeds directly into the soil, till the upper inch of soil to loosen it for direct seed to soil contact. The outside soil temperature must be above 50 degrees for cool growing plants, and at least 65 degrees for warm season annuals, according to Prosek.

For future reference, keep a record of indoor sowing dates, germination dates, length of indoor growing period, and transplanting dates.


What are some good plant/tree choices to hide unattractive views?

I’d like to block an unattractive view of my neighbor’s house/yard. What are some good plant/tree choices to hide unattractive views?

John Eskandari, owner, The Urban Plantsman, Chicago

Two evergreens for sun are Thuja occientalis ‘Wintergreen’ and Picea pungens ‘Fastigiata’. Both are moderate to fast growers. For shadier locations, Picea abies ‘Cuppressina’ does remarkably well.

A deciduous tree that would work nicely is Fagus sylvatica ‘Dawyck’. It comes in green, gold, red and purple leaved varieties. They work well in shadier spots but the red and purple leaves are less intense in shade.

In smaller areas, I like Amelanchier laevis ‘Cumulus’. It’s a more upright form of serviceberry providing nice spring blooms, edible berries, and golden fall color. It is adaptable to both sun and shade.

Chrissie Sieff, nursery manager, Platt Hill Nursery, Bloomingdale

Whenever I am asked this question, my customers usually have something in mind such as ‘Emerald Green’ arborvitae (Thuja occidentalis ’Emerald Green’, sometimes labeled ‘Smaragd’). They want a green wall, and they want it instantly! Our dead plant return rate is high on this item, so I try to steer them to hardier choices, such as Viburnum nudum ‘Winterthur’ or Syringa pubescens subsp. patula ‘Miss Kim’.

A strategically placed tree can also do the trick. I love katsura tree (Cercidiphyllum japonicum), river birch (Betula nigra), serviceberry (Amelanchier), and ‘Ivory Silk’ tree lilacs. As you can tell, I’m not a big fan of evergreens.

Andy Wedel, part owner, Wedel’s Nursery, Florist & Garden Center, Kalamazoo, MI

Arborvitae (Thuja spp.) is the first plant we think of, but there are others. It’s hard to pick just one. Fine Line buckthorn (Rhamnus frangula ‘Ron Williams’) is a columnar-shaped deciduous shrub with yellow fall color. Evergreen, drought-tolerant upright junipers such as Juniperus chinensis ‘Blue Point’ are great in sunny spots. Some other columnar trees include ‘Crimson Sentry’ maple (Acer platanoides ‘Crimson Sentry’), Red Fox katsura (Cercidiphyllum japonicum ‘Rotfuchs’), and columnar hornbeam (Carpinus spp., cvs.).


Variegated Sedum

I have a large variegated sedum with pink flowers that I have had for years. I noticed that it has started to send up some all-green shoots. Why is it doing this and how can I keep my plant variegated?

All variegated plants contain a combination of green cells with chlorophyll and white tissue with no chlorophyll, which produces the green and white effect. Variegated plants are genetically unstable, so sometimes the tissues revert to all-green cells and send up solid green shoots from the base of the plant. Because these shoots contain more chlorophyll than the variegated tissue, they are more vigorous. Thus they can quickly overrun a plant, according to Hortech, a developer and grower of sedums.

Cut the green shoots off at the base of the plant as soon as you see them. It is possible you have induced vigorous growth by giving the plant too much high-nitrogen fertilizer, so it would be wise not to fertilize. It sometimes helps to divide your plant and then choose the most variegated stems for a new plant. Since your sedum, a cultivar of Sedum spectabile, is such a rapid grower, it will quickly become a new clump after division. Newer named cultivars are more genetically stable and do not have this problem.


Cold Frames

With all the emphasis on growing fresh vegetables, I think I should use a cold frame but I am not sure what to do or how to go about it. Any ideas?

Cold frames are simply four walls with a window or plastic sheet on top. They range from simply placing four bales of straw in a square with an old window atop them to more deluxe mini-greenhouses with aluminum frames, plastic panels and automatic vents using solar energy to open and close them automatically.

Before building a cold frame, study garden supply and greenhouse catalogs to determine your needs and how fancy you want your cold frame to be. Do you prefer a permanent installation or an easily disassembled frame?

A basic plan for building a cold frame is taught by Chicago Botanic Garden’s senior horticulturist Heather Sherwood. Create four walls from plywood or bricks. Add a transparent lid (plastic sheeting or an old window) for light to enter. Place the lid on an angle. Place the frame against a building, facing south. Make sure there is good drainage. Open the frame when it gets above 60 degrees. Rabbits don’t usually get into the box but chipmunks may squeeze in.

Cold frames are used for extending the growing season, starting plants early, for storing bonsai or protecting semi-hardy plants over winter. The soil in the frame depends on how you will use it. The frame can be set on existing soil when you are storing plants, or fill it with straw to protect bonsai. Locate it conveniently against the house near a back door for easy access, especially if you envision running out to cut some salad greens when snow is on the ground.

Use a cold frame to get a head start on the gardening season. In early spring, start flats of lettuce, radishes, parsley, pansies and other cool-weather plants in the house, and place them in the cold frame in March, depending on the weather. If you plan on planting directly into the cold frame, fill it with a seed starting soil mix and an inch of compost worked into the top 3 inches of the soil. This will help keep the soil loose and aid moisture retention. The plants will be small in the cold frame and can be grown close together. Also use the cold frame to harden off house-grown seedlings.
Don’t forget to check on your plants regularly because winter sun is strong and plants dry up easily. Use it for a head start on the gardening season.


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I’d like to block an unattractive view of my neighbor’s house/yard. What are some good plant/tree choices to hide unattractive views?

At the end of every winter, there are many shrubs growing along sidewalks that are dead and damaged either by salt, wind or dogs. Are there any shrubs that I can plant in these more exposed situations and expect them to survive?

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