Holiday poinsettias: Keep the plant in bright light but slightly cool conditions. Don’t site poinsettias near fireplaces, heaters, or other areas where warm, dry heat is blowing directly on the plant; this causes leaf drop. Don’t overwater — plants that are too dry or too wet also drop leaves. If the pot is wrapped in foil, punch holes in the bottom of the foil so the water can drain out. Set the pot on a saucer. Plastic saucers are best —clay saucers can collect moisture underneath them and damage your furniture. Generally, fertilizer isn’t needed if you are only keeping your poinsettia into February.
Poinsettias can be kept from year to year and “re-bloomed” but it requires adherence to a schedule. If you’d like to try it, here is what to do. After the holidays, keep growing the poinsettia as a houseplant, keeping it evenly moist but not wet and in bright light but not direct sun. In mid-March, cut the flowering stems back to four to six inches tall to promote new growth. Apply a granular slow-release fertilizer in April. Most of these products last 3 months, so check the label — when three months are up, start using a 15-15-15 or 20-20-20 water-soluble fertilizer every two weeks to finish out the summer.
In May, repot into a container no more than two inches in diameter larger than the old pot. Leave a half-inch to an inch of space between the top of the soil and the rim of the pot as a water reservoir for when you are watering. Place the plant in a sunny window (east or west-facing) until after the last frost. Night temperatures need to be above 60 degrees. Then, the pot can go outside for summer in an area where it gets morning sun or dappled sun all day. You may sink the pot into the ground or set/sink the pot in a larger decorative pot so it doesn’t dry out. If needed, rotate the pot often to keep the shape rounded.
In June, start shaping the plant for height and “flowering” branches. For a short, dense plant with many stems bearing flowers, pinch out the top one-quarter inch of the tips of the growing shoots to encourage more branching. Do this every three weeks. Retain two or three large fully expanded leaves below the pinch so you can use their size as a guide for knowing when the shoots are ready for pinching again. Continue this until mid- August, as the plant should by then have a good shape and number of shoots. Check the plant periodically throughout summer to see if it needs repotting again — watch for a dense network of roots on top of the soil or a number of roots coming through the drainage holes in the bottom of the pot. If you see either of these, repot by the beginning of August.
As September arrives, bring the plant in before night temperatures fall below 55-60 degrees and place indoors in a sunny location. Continue watering but reduce the fertilizer to once a month and discontinue it in October.
Flower production is stimulated by long nights. Without them, poinsettias won’t flower. If your plant is in a room that is lighted nightly, cover it completely at 5 p.m. every day starting in the middle of September with a box or other light-eliminating cover, or place the plant in a dark closet overnight. Light should not come in under the door. In order to get poinsettias to flower by Christmas, start the “long night” treatment in the middle of September and continue until Dec. 15 or so. Dark periods longer than 12 hours are needed for flowering. So, for best results, plants must be in total darkness from 5 pm to 8 am.
However, poinsettias need light during the day to grow, so pull them out of the box or closet each morning. Once you see the small yellow “true flowers” at the stem tips developing and the bracts (what you may think of as the colorful “petals”) show color, it isn’t as critical to continue with long nights, though it is best to continue until the bracts are almost fully expanded.
Keep night temperatures no lower than 55 degrees, and not more than 70 degrees. High night temperatures, coupled with low-light intensity, low nutrition, dry soil or improper photoperiod may delay flowering, so keep an eye on those conditions as well. Good luck with your plant!