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Fielding Questions: How can I coax my poinsettia to bloom?

This week, Don Kinzler addresses how to make a poinsettia bloom, whether herbicide-treated yard clippings are safe for compost and when to remove the stakes from a new tree.

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This reader's poinsettia plant refuses to bloom.
Contributed / Hope W.
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Q: I have a poinsettia plant that I’ve kept alive for three years. It’s beautiful, and spends the summers outside. Is there a way to force it to bloom? - Hope W.

A: The poinsettia looks wonderfully healthy! The inner leaf-like "bracts" of poinsettias turn color in response to shortened daylength, which happens naturally, of course, as our days get shorter. That’s why in their native frost-free outdoor habitat they bloom during the short days of the winter season.

Poinsettias will usually bloom on their own indoors at some point during winter’s long, dark nights, but only if they don't receive indoor light from a room, which makes the poinsettia think the days are still long, and prevents flowering.

If the poinsettia is in a room that stays dark all evening and night, they'll begin to color up on their own sometime in December or January. To achieve bright color for the Christmas season, commercial greenhouse producers "force" poinsettias to bloom by late November or early December by providing long dark nights ensured by blackout curtains or other devices, so no extraneous light enters the greenhouse.

We can follow the pattern of poinsettia growers, and force the plants to bloom by Christmas in our homes by giving the plant darkness starting about 5 p.m. each evening until around 7 or 8 a.m. the next morning beginning late September until they are well-colored.

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The long, dark night can be accomplished by moving the plant into a dark closet each evening and out again the next morning to a sunny window, or covering with a black garbage bag or box each evening and uncovering the next morning. It’s important to give the plants their daily time in a sunny window. The daily cycle of pitch black dark nights and sunny days continues until the plant is well-colored, then the plant can be treated like any other houseplant.

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Q: I have a barrel of grass clippings from a neighbor. Their lawn was sprayed for weeds, and I know you’ve said not to use such for mulching or in compost. Is there a time period after which the chemical would be inactive and the clippings could be safely used for mulching or composting? - Martha B

A: The length of time herbicides persist on lawn clippings depends on what herbicide was used. Some herbicides break down very, very slowly on material to which they’ve been applied, and such persistent herbicides can retain their plant-killing ability on grass clippings for years.

Herbicides used on lawns are termed “broadleaf herbicides,” formulated to kill weeds such as dandelions without harming grass-type plants. Herbicide residue that persists on grass clippings can damage “good” broadleaf-type plants such as vegetables and flowers, if the grass clippings are used as mulch on gardens and flowerbeds. Some herbicides even persist in compost created from lawn clippings.

Although some lawn herbicides break down more rapidly than others, the product applied is often unknown and that’s what makes using herbicide-treated grass clippings so unpredictable and potentially damaging. To prevent the heartbreak of incorporating residue of damaging chemicals into gardens, it’s generally not recommended to use herbicide-treated lawn clippings as mulch or incorporated into compost.

Q: Our buckeye tree was planted and staked in 2021. I kept the stakes in place this year after the crazy windy spring we had. I’m wondering if it’s time to remove the stakes and let it stand on its own. What do you recommend? - Dean H.

A: The usual recommendation is to leave the stakes in place for one growing season, two at the most, and then remove them. It sounds like it’s time to remove them from your buckeye, which, by the way, is a great tree choice for the Upper Midwest.

Winter winds are usually less of a factor on the bare branches of a tree when foliage is gone, so you might wait to remove the stakes when leaves have fallen after a few frosts.

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If you have a gardening or lawn care question, email Don Kinzler, NDSU Extension-Cass County, at donald.kinzler@ndsu.edu . Questions with broad appeal may be published, so please include your name, city and state for appropriate advice.

Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at donald.kinzler@ndsu.edu.
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