Thomas Christopher | Be-A-Better-Gardener: Repurpose tropical houseplants into your summer landscape

STOCKBRIDGE — Early in my gardening career, one mentor, an octogenarian English horticulturist, observed that New York has the climate of Siberia in the wintertime, but of Bangkok at mid-summer. An exaggeration, of course, but it included more than a grain of good sense. Most of the Northeast doesn't sweat the summers like New York City does, but still our combination of heat and humidity at that season can be formidable. And the smart gardener will take advantage that fact.

After all, if the weather is going to be tropical, why not plant tropical? The alternative is not so successful, at least at mid-summer: when the dog days settle in, our hardy perennials and annuals sulk, typically pausing in their bloom until the weather cools again. But the dog days are when plants of tropical origin really shine, for they just get better and better as the summer weather grows more intense. These include a number of warm-climate perennials, such as impatiens, coleus and lantana, that we commonly grow as annuals. But we shouldn't ignore the contribution that our houseplants can make at this time of year. Many of the tropical foliage plants that are mainstays of indoor displays flourish if moved outdoors in the summer. Transfer them from their pots and into the soil, and they can add a notably lush note to your beds and borders.

This isn't a new idea, by any means. A couple of weeks ago, I was in the library of the New York Botanical Garden and I found a book by a man who was arguably the first American landscape architect, Andrew Jackson Downing. In a book published in 1842, "Cottage residences: or, A series of designs for rural cottages and cottage villas, and their gardens and grounds, adapted to North America," Jackson promoted the use of "exotics," such as verbenas and zonal geraniums, praising "their many varieties, their brilliant colors, and their power of withstanding heat and dry weather, [so that they] have done more to give an air of perpetual beauty to our flower gardens than all other plants together."

So useful can these indoor/outdoor plants be that I'd suggest adjusting your choice of houseplants to promote this second usage. Some of the smaller types of banana trees, for example, such as `Truly Tiny' and `Super Dwarf Cavendish' are compact enough that they can be accommodated in a sunny room and yet make a fine centerpiece for an outdoor display during the warmer weather. Other tropical foliage plants ideal for this purpose include dieffenbachias, spathiphyllums and philodendrons. Cannas (Canna x generalis) are usually treated as outdoor annuals, but the more compact types, such as the "Canova" cannas, which were bred to flourish in cooler conditions and lower light, can make eye-catching houseplants as well.

You do need to observe a couple of precautions when moving your indoor plants outdoors. Don't try this transfer until daytime and nighttime temperatures stay above 50 F. Nor should you abruptly move plants from the relatively low light conditions of indoors to a full-sun situation outdoors, even if the plants are recommended for such. Shade-loving houseplants should go into a shady location outdoors; sun-lovers should go in the dappled shade underneath a high tree canopy for a week to acclimate them to the outdoors before you move them to their final, sunny, location.

Fertilize your house plants with a half-strength solution of some balanced, water-soluble fertilizer when you transplant them from their pots into the garden beds and fertilize with the full-strength solution at least monthly thereafter until fall shuts down their growth.

When the night-time temperatures threaten to drop below 50 F., dig the houseplants from their position in the beds, and repot them into their containers. This is likely to require pruning of both the tops and the roots, as the plants are likely to have made considerable growth over the summer. In some cases, you may find it better to start fresh with cuttings in the fall, growing them on over the winter to set them out again the following summer. Every houseplant should have a second life as a mid-summer garden star.

Be-a-Better-Gardener is a community service of Berkshire Botanical Garden, located in Stockbridge, Mass. Thomas Christopher is the co-author of "Garden Revolution" (Timber press, 2016) and is a volunteer at Berkshire Botanical Garden.


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