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LifestyleColumnistsJessica Damiano

Garden Detective: Hardening off greenhouse plants; gardenia care

Linda and Bob Cavaliere of Port Jefferson Station

Linda and Bob Cavaliere of Port Jefferson Station keep their plants in a greenhouse over winter, a factor worth considering when plants are moved outdoors. Credit: Bob Cavaliere

DEAR JESSICA: I was a little confused after reading your Feb. 16 column about hardening off hibiscus plants. We have a hibiscus that stays in our greenhouse over the winter, and it gets put out in the spring. You said we should place it outside in the shade for an hour, then return it inside. Then we're to increase the shade time by an hour each day. It seems strange to increase shade time over sun time. Can you please explain again? — Linda and Bob Cavaliere, Port Jefferson Station

DEAR LINDA AND BOB: The reason for “hardening off” indoor plants is not only to gradually accustom them to sunlight, but to wind and temperature fluctuations.

Because you have a greenhouse, and the plants already are in direct sunlight, you could start the process by opening the windows for a few hours during the day, if that’s possible, for a few days. Next, move plants to a sheltered outdoor spot for a two hours, then three or four the next day, using your judgment to adjust for weather conditions (sunny, cloudy, windy, etc.). I’m sure this will be a relief, given the number and sizes of your plants.

Moving any plant straight from the indoors to a sunny outdoor spot — even for just an hour — risks stressing, shocking or scalding it (similar to a person's first beach trip of the year without sunscreen or at least a hat). In fact, after spending extended time indoors, a plant requires gradual acclimation even to a shaded outdoor spot.

I don’t recommend hardening off plants in deep shade, however, but rather a spot that’s sheltered from sunlight. Every spring, for instance, I harden off a tray of seedlings by setting it under my patio table. Under a hedgerow, bush or tree would work, as would providing protection with shade cloth or row covers. The idea is to provide a "beach hat" to filter the sun.

The difference is that with plants, after a while, it’s safe to take the hat off altogether and forgo the sunscreen.

DEAR JESSICA: My sister sent my mom a gardenia plant, but it didn’t come with directions. I am watering it, yet the leaves are starting to turn yellow. It is in a bright room away from direct sunlight. How should we care for this plant? — Stephanie Fioretta, via email

DEAR STEPHANIE: Gardenias (Gardenia jasminoides) are beautiful and beautifully scented, but they have a well-earned reputation as divas in the houseplant world. You’re doing well providing bright, indirect sunlight, but these are subtropical plants, and as such prefer a subtropical climate.

For starters, they require a lot of humidity. This is best achieved by running humidifier indoors near the plant, especially during winter, when heating systems tend to dry out the air. If that’s not possible or practical, set the plant’s pot on a shallow tray filled with pebbles and water. A rimmed baking sheet works well for this, but you can use a more decorative option, if you prefer, or even a large plastic plant saucer. Cover the pan’s bottom with pebbles or glass beads and keep the water level deep enough to reach three-quarters of the way up the pebbles, but not so deep that they float or that the water can enter the pot’s drainage holes. As the water evaporates, it creates a microclimate around the plant that provides the humidity it requires.

Gardenias also are heavy feeders. Apply an acid-based fertilizer every two weeks (except during winter), following package directions for dosing guidelines. In addition to providing nutrients, the acid will help keep the soil at the desired 5.0 to 6.0 pH range and prevent an iron deficiency, one cause of yellowing foliage.

Keep the soil consistently moist, but never soggy, as too much water can result in root rot, and — you guessed it — yellow foliage. Once a month, water with distilled water to flush excess salts from fertilizer that may accumulate in the soil. Always allow excess water to drain from the holes in the bottom of the pot, and empty the saucer (or adjust the water level in the tray) when water accumulates (plant pots should never be allowed to stand in water).

Perhaps the most difficult condition to attain in the home is the temperature range these plants require for blooming. Gardenias prefer daytime temperatures between 65 and 75 degrees, easy enough to provide indoors year-round; to set buds and flower, however, they require cool nighttime temperatures, ideally 55 to 62 degrees, and never over 65, conditions that can be difficult to achieve.

Gardenias also are susceptible to mealy bugs, aphids, spider mites and scale insects, so keep an eye out for pests. Inspect plant parts, especially the undersides of leaves, regularly for cottony masses, raised bumps or webbing.

Root nematodes, tiny wormlike organisms that take up residence inside roots, are yet another cause of yellowing foliage. Unfortunately, there is no control for them: Infested plants should be discarded.

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