Regional Gardening Reports :: National Gardening Association

In the Garden:
April, 2005
Regional Report

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Let petunias brighten your doorstep and welcome you home.

Sunny Window Boxes are Hot!

Sunny window box planters are fabulous early in the season. They are irresistibly adorable in the early spring sunshine, sparkling with scented hyacinths or spritely crocus. By mid-spring the cheery arrangements boast colorful pansies and snapdragons, sparkling lobelia and fragrant sweet alyssum. But, by midsummer, those spring charmers will be quite literally toast if they face south or west.

Wise gardeners realize that super sunny window boxes -- those that face west or south -- become heat traps in midsummer. With sun reflecting off the house and beating against the exposed side of the box, not only does the soil heat up and bake the roots, it also dries out fast. And as the season progresses and your plants get bigger, they'll need more water. This means you need to take steps to help your plants survive the heat of summer.

First of all, the larger the volume of soil, the better. Many pre-made window boxes are just too small to support mature-sized annuals. When the roots become too crowded, they lack water and nutrients and the plants suffer and show symptoms of stress: mid-afternoon wilting, stalled growth, and lack of bloom. Under extreme stress, they just plain die.

Select plants that like hot temperatures, warm soil, and ample sun. This seems like a no-brainer, but in the gentle spring weather it can be hard to think that far ahead. Look for plants that like full sun and are also somewhat drought tolerant, such as marigolds, geraniums, salvia, petunias, callibrachoa, verbena, helichrysum, artemesia, sun coleus, dracaena spikes, sanvitalia, argeranthemum, plectranthus, thunbergia, lantana, celosia, melampodium, portulaca, dwarf cosmos, Zinnia linnearis, and Catharanthus roseus (flowering annual vinca). Your local professional garden center staff may have more suggestions based on what is available in your area.

Among your selections, try to include a trailing plant (e.g., verbena, thunbergia, callibrachoa, or helichrysum) or one that can hang down and shade the front side of the window box. This will help so much in keeping the soil cooler and also, as a corollary, moister.

Good Roots Make Great Plants
Your window box should have drainage holes in the bottom to allow excess moisture to drain, such as during a rain storm, and your soil mix should be a good-quality commercial mix formulated for container growing so that it holds both air and water like a sponge. Garden soil and topsoil are not recommended because they tend to compact quickly and do not drain well enough.

Watering is as much art as science. Early in the season you will not need to water as much as you will need to later when the weather is hotter and the plants are bigger. To know if you need to water, dig into the soil with your finger. If it is still damp, don't water yet. If your plants are wilting in the afternoon, it may be due to heat stress and they will perk up by evening. If they are wilted in the morning, they are definitely overdue for water. When you water, make sure it soaks into the soil and moistens it thoroughly and doesn't just dampen the top inch or immediately run out the sides between the soil and the container.

Prior to planting, you may want to mix in one of the water-holding polymers if your soil mix does not already include it. This should help the soil retain additional water that plants need on those long, hot, summer afternoons. A slow-release fertilizer added at planting time also can be helpful in keeping plants going strong all summer and into the fall. In addition, I like to use a water-soluble fertilizer (such as 10-10-10 plus minor nutrients or sometimes a kelp-based formula) for an occasional boost, and I periodically add some good-quality compost on top of the soil.

Finally, grooming is important. Deadheading or removing dried, faded flowers keeps the plants looking spiffy and also saves their energy for more blooms instead of seed production. Some plants also may benefit from a mid-season trim and extra pick-me-up feeding to reinvigorate them for a late-season flush of new growth.

Smart plant choices and regular care will turn your hot, sunny, window boxes into show-stoppers all season.

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