Consider window boxes as great container gardening options

Story and Photos by Tom Hewitt

Many years ago I wrote an article about window boxes for Florida Gardening. Though the basic rules remain the same, the wide selection of plant material available today means we’re long overdue for an update. I still insist window boxes would be much more popular down here if people knew how easy it was to make them.

Monochromatic combos are always a safe bet.

The primary nemesis of traditional wooden boxes is constant moisture. That’s why I often replace the wooden bottoms with wood slats to facilitate drainage. Then I just set the pots directly in the box; soil never touches the box itself. You could put plastic liners in wooden boxes, or simply place a plastic window box inside a wooden one. Don’t bother making one using cheap white pine. No matter what you do, they won’t last long.

Wooden boxes aren’t our only option today. Those made of polyethylene and other composite materials often resemble real wood, but never rot. Metal trough boxes with coco liners are also some of my favorites. I also like terra-cotta, though I usually relegate these to my back porch or patio because of their weight.

Window boxes in shady areas rely more on colorful foliage than flowers.

Speaking of weight, I strongly suggest using potting mix as opposed to potting soil. Soilless mixes drain much better than soil. And don’t bother with a window box less than 8 inches deep. Plants grow quickly down here and our growing seasons are long. Don’t waste valuable growing space by putting stones or pottery shards in the bottom. Cover drainage holes with coffee filters instead.

Be sure to incorporate a generous amount of balanced, slow-release fertilizer at time of planting. If things need a boost, you can always apply a liquid blossom-boosting fertilizer at half strength every 10 days or so. 

‘Marguerite’ sweetpotato vine is the focal point of this colorful window box.

The biggest mistake most people make with window boxes is combining the wrong plants. Make sure they all have the same lighting and irrigation requirements. Most importantly, make sure they have compatible growth habits, as you don’t want one plant bullying another.

A good rule of thumb is to choose shorter versions of your favorite plants for window boxes. Tall summer snapdragons (Angelonia spp., cvs.) grow way too big, but smaller hybrids are perfect and bloom nonstop. The same can be said for strawflowers (Bracteantha bracteata) – dwarf selections, such as Tom Thumb and Bright Bikini mixes are much better choices.

Million bells work much better than petunias, especially in South Florida.

French marigolds (Tagetes patula) work better than African marigolds (T. erecta), and more compact varieties of fanflower (Scaevola aemula) such as Brilliant (‘Scalora Brilliant’), work better than standard varieties. Pot marigolds (Calendula officinalis) and snapdragons (Antirrhinum majus) also come in shorter varieties, and many newer Verbena and nasturtium (Tropaeolum majus) cultivars stay much more compact. Here in South Florida, I gave up on growing Petunia in window boxes years ago, but now I plant million bells (Calibrachoa hybrids), which are much more compact, and last longer.

Mixing several varieties of one particular plant make lovely window boxes. Zinnia are my favorites, since you can grow them any time of the year. Put larger zinnias (Z. elegans) in the background and smaller, narrow-leaved varieties (Z. angustifolia) in front. You can fill an entire box with plants from the Profusion series, which now come in apricot, cherry, orange, yellow, and white. Sow seed directly in the box if you wish, and thin them out later. Tuck some nasturtium seed in front for added interest.

Make sure you use compatible plants so one doesn’t outgrow another.

You can grow almost anything in widow boxes during the winter, but during the summer, I rely on succulents for color and interest. Succulents need grainy soil that drains well to keep prevent from rotting. If your soil is on the heavy side, be sure to go easy on the watering, especially during the rainy season.

Good cascading succulents for window boxes include baby sun rose (Aptenia cordifolia), moss rose (Portulaca grandiflora), various Sedum species and cultivars, and ice plant (Delosperma spp.). For upright interest, use Echeveria, Kalanchoe, Aloe, and dwarf varieties of snake plant (Sansevieria spp.).

Boxes in shade rely more on colorful foliage than flowers. Good fillers and bloomers for shady combos include coleus (Plectranthus scutellarioides, syn. Solenostemon scutellarioides), sweetpotato vine (Ipomoea batatas), Lobelia, wax begonias (B. x semperflorens-cultorum), Torenia, creeping jenny (Lysimachia nummularia), English ivy (Hedera helix), Swedish ivy (Plectranthus australis), Persian shield (Strobilanthes spp.), and polka dot plants (Hypoestes phyllostachya).

Mediterranean herbs mix well in window boxes.

Herb combos can be fun, but take care to combine the right ones. Mints (Mentha spp.) and many others don’t play well with others, but many Mediterranean herbs are highly compatible. These include varieties of oregano (Origanum vulgare), thyme (Thymus spp.), rosemary (Rosmarinus officinalis), marjoram (Origanum majorana), and Salvia. And don’t overlook adding veggies to combos, like ornamental kale and small varieties of hot peppers.

Remember to deadhead plants to keep your boxes looking their best. Choose wisely, and you’ll be rewarded with months of pleasure for thinking inside the box.

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