Q: How do I get my poinsettia to bloom again? It looks fine, but the leaves are all green.

A: You are a month late in starting the process, but the poinsettia should still be able to deck itself in holiday color by the end of the year. To do so, it needs long nights of absolute darkness. Commercial greenhouses use 15 hour nights, but anything over 12 hours will suffice.

Poinsettias are exacting about what constitutes night. As far as they are concerned, if there is any visible light, it is only twilight. There is a classic story about an inexperienced employee in a greenhouse full of poinsettias. One night he accidentally hit the light switch. By the time he turned it off again, it was too late. All the poinsettias stayed green for that holiday season.

For a poinsettia in a home, the easy way to give it night is to cover the plant with a big box. A double thickness of black plastic bags will also suffice; a single bag does not keep out all light. Some poinsettia owners find it easier to move the plant to a closet or an unused room every night.

One way to schedule poinsettia nights is to have them begin at suppertime and end at breakfast. But skipping a night even once will break the schedule. One year I forgot to cover a poinsettia until I went to bed, so I left it in the dark all the next day. It accepted that strange break in timing and continued turning red.

Meanwhile, a poinsettia needs to spend its daylight hours in the brightest light your house has to offer; a south window is perfect. After a week or two of its day-night regimen, dark red patches will appear on the leaves and gradually spread. When the poinsettia is fully dressed for the holidays, stop the nighttime covering. It will keep its color for a month or more. Do remember to give it plenty of water while it is changing color and making its tiny yellow flowers.


Q: I read that I should mow the lawn shorter before winter. Is that right?

A: That is information for climates where humidity is high, rain is frequent, and grass is susceptible to fungal diseases. In this dry climate I recommend mowing at the usual height — two and a half to three inches — before the grass goes dormant for winter. The brown leaves left in place will protect the roots from winter cold.


Q: How should I prune my climbing roses, and when? They are far too tall, and I have to do something. Once I followed a neighbor's advice and cut them to the ground in the fall. I won't do that again.

A: I belong to the school of pruning hardy roses when the daffodils bloom. By that time the worst of the winter weather is over. Pruned in fall or late winter, roses can be unintentionally pruned a second time by cold weather that freezes more stems.

Where should you make the cuts? Try cutting the longest, most awkward stems about 18 inches above the ground next spring. Do not cut more than half of the total number of stems. In the second spring, repeat this pruning, cutting back all the remaining stems which are too tall. (Of course a dead stem should always be cut down entirely.) In succeeding years you will be able to keep the roses at the height you want simply by cutting back any remaining stems which are too long. Do not trim the tips off stems, because new stems will start from the point where you pruned the old ones. That would create even taller roses.


Master gardener Molly Hackett, whose motto is “Never trust a gardener with clean fingernails,” welcomes your questions. Send them to 1384 Meridian Road, Victor, MT 59875; call 961-4614; or email mhackett@centric.net. Please include a garden-related subject line in emails. Hackett writes a twice-monthly Dirty Fingernails opinion piece for the Missoulian.