Holiday horticulture hints: How to keep a good thing growing

Pity the poor poinsettia, welcomed into our homes in December as a bright symbol of holiday spirit, only to be tossed in the trash when the new year arrives.

Christmas cactus, amaryllis and living holiday trees like Norfolk Island pine also are popular at Christmas. But what do you do with them once the flowers fade and the holiday is a memory, leaving behind a mystery plant?

The good news is, all these plants can thrive in the right conditions. In our dry and drafty winter homes, poinsettias can be the pickiest of the bunch, followed by Norfolk Island pine. Amaryllis, though, will easily rebloom for many years. And in some families, Christmas cacti are heirloom plants, handed down for three generations and still going strong.

It's all about mimicking the light, temperature and seasonal patterns that these plants experience in their native habitat. Here's a rundown on how to keep them going.

Kept in less-than-ideal conditions, poinsettias can turn into a limp mass of shriveling, dropping leaves. Remember that the first cultivated poinsettias were tropical plants from Mexico. They grew in bright light, warmth and soil that was moist but not sodden, and enjoyed the same conditions when they were grown for sale in greenhouses. If you want to keep your poinsettia going, you need to reproduce those conditions as best as you can.

Put the plant in bright light, but avoid cold drafts or dry heat from vents and radiators. A warm spot by a window is ideal, but don't let the leaves touch cold glass. Experts recommend growing poinsettias where the daytime temperature is 65 to 70 degrees, moving the plant to a cooler spot at night. Poinsettias will sulk if the temperature drops below 55 degrees.

Watering is key. A dry poinsettia will dramatically wilt, but one grown in saturated, cold soil will rot. I remove the decorative foil that usually covers the pot, to make sure there is a drainage hole in the bottom. If there is, put the pot on a saucer to catch any excess water. Water your plant with tepid water every few days when the soil seems dry, but never let it sit in water.

As fall approaches, bring the poinsettia inside to a sunny location. In mid-September, begin putting the plant in total darkness for 14 hours each night, moving it back to its sunny place during the day. Putting the plant in a closet or covering it with a black trash bag works. Stick to this routine religiously until the flower bracts at the top of the plant turn color, usually in about eight to 10 weeks.

Christmas cacti aren't as fussy as poinsettias. Oddly, many of the flowering cacti that are sold as Christmas cactus are actually Thanksgiving cactus. Though the plants are related, you can tell a Thanksgiving cactus by its jagged saw-toothed stems. Stem segments on Christmas cacti are more rounded.

The two need the same conditions to thrive. They like bright light in fall and winter but light shade in summer, when they can be put in a sheltered location outside. Bring them back inside before frost. The plants set bud in response to shorter days and cooler nights.

Here's a fuller explanation of how these Brazilian natives can thrive and reliably flower in Minnesota: extension.umn.edu/county/benton/county-horticulture-educator/article/why-does-my-christmas-cactus-bloom-thanksgiving.

Amaryllis should be planted with the top third of the bulb exposed. After the flowers fade, snap them off but let the flower stem wither and dry before removing it; this gives energy to the bulb. The strappy leaves will persist after bloom, and you can treat the bulb as a houseplant until spring comes.

The U of M has a nice fact sheet with more information on amaryllis here: extension.umn.edu/garden/yard-garden/flowers/growing-and-caring-for-amaryllis/.

This year I saw beautiful Norfolk Island pines for sale in some big-box stores in the Twin Cities. Some gardeners manage to keep these gorgeous trees, which are often sold as tabletop Christmas trees, for many years.

Again, the secret to success is to understand where these plants came from. They are tropical in origin. Bright light and high humidity are a must for Norfolk Island pines. They prefer about 50 percent humidity, which is definitely a challenge to provide during a Minnesota winter.

In summer, they should be watered enough for the soil to remain moist, but in winter, water only when the soil is dry. The plants hate wet feet, so the pot should never sit in water.

Grown in less-ideal conditions, the plants will have browning branches and may be subject to pests like spider mites, aphids and scale, which can be tough to shake.

Mary Jane Smetanka is a Master Gardener and Minneapolis freelance writer.