Growing Concerns 603: Jasminum polyanthum, silver maple, baltic ivy


Be smart in the fall or work hard in the winter to make a jasmine bloom

Dear Janet,

For Valentine's Day last year my husband gave me a pink flowering jasmine (Jasminum polyanthum). I would like to get this plant to flower again, although I am very much a beginner when it comes to gardening. So if it's too complicated I have decided it's okay to buy a new one, beautifully in bloom.

One website I read suggested this plant needs an 18 degree temperature drop to flower. Is it true?

Well, if it is true, I found a cool closet in my home that's around 50 degrees. Do I put her in only at night, or all day with lights? Gradually, or start tomorrow? How many days or weeks is necessary for her to get flower buds?


 Dear D.C.,

Many plants of temperate regions need to experience a change of weather and daylength to "know" that a new growing season is coming. Because of cooler air and longer nights at that change of season, different chemical reactions take place in the plants' cells. When the chemicals that are the by-products of those reactions reach adequate levels, flower buds form.

Jasminum polyanthum is such a species. It can't take Michigan's frigid winters but it does "expect" a season that's at least 18 degrees cooler on average than the summer. So it might flower as well in Louisiana or southern California as it does in its native southeast China, but not in Miami where temperatures are too steady all year. In Michigan you can induce bloom if you leave it out in fall as nights cool toward 40 degrees. Although we must bring it indoors before its foliage is frosted or its roots killed back, it may get enough weeks of good growing at lower temperatures to set buds.

How many weeks depends on how well the plant is growing and how mature it was before it began being chilled. Authorities won't name a number, only say that once it begins to set flower buds, then you can stop the special treatment. Those buds will ripen and open as long as the plant continues to have very good light and steady moisture.

It's possible that cool nights alone may do the trick, but I can't find any data to support it. Growers in a position to work with statistically valid numbers of jasmine don't move them each day. They do the simpler thing, dropping the temperature of the greenhouse overall, all day and night.

So it's not complicated, just challenging, to induce another round of bloom in your jasmine once that natural opportunity in fall has passed. To simulate those long nights and cool weather, you might set the plant under grow lights in your cool closet. Use fluorescent lights, to avoid warming the closet. Put them on a timer for twelve hours on, twelve off. Check your plant regularly to keep it watered. Four to six weeks of that may bring out the bloom.

Or you can try the nights-only treatment. If you do, let us know how it comes out.


Dear Janet,

I have a 55 year old silver maple in a bed of Baltic ivy. The ivy has grown up the tree and is very attractive, but a couple of my children are insisting that the ivy will kill the tree if I don't remove it. Is this true? It took several years to get the ivy established and I'm fond of the effect.


 Dear J.D.,

Evergreen ivy (Hedera helix) won't hurt a tree's trunk or main limbs. It's not a strangler, like bittersweet vine. Its roots don't penetrate the bark to put the sensitive cambium at risk. In fact, evergreen ivy provides an extra layer of protection from cold and attract small birds that while sheltering there may put a dent in your insect population.

Do your children garden in milder climates? In the Southeastern U.S. or California, their advice may be sound. There, unlike in Michigan where winter kill works in our favor, ivy can bridge the gaps between trunk and limb, and scramble right out to the branch tips.

Ivy crossing between limbs turns the tree into a huge sail. That tree catches more wind and water than it should, and may topple. Meanwhile, ivy growing out to and over the tips of the branches can shade out the host tree's foliage and kill it.

Here in Michigan, rest easy if you have ivy on a tree and like the look. It will stay on the trunk and main limbs, unable to survive the exposure it gets in winter in the open areas between limbs and out toward the small branches.


Some problems have no solution. I can't provide much help to those who pose a "stumper" such as:

Why is it that a plant described by knowledgeable authorities as reaching "ten to twenty feet" will stop growing at ten feet if I wanted 20, but will top 20 if I chose it to grow in a restricted ten-foot space?


Green thumbs up

to Groundhog Day, the turning point of winter. It should be a day of thumbs up and relieved smiles between gardeners.


Green thumbs down

to neglecting the birds during big snows. You benefit even in winter from the nit-picking they do that helps you control insects in your yard. So keep the birds alive. Shovel out that feeder and keep it filled.

 Originallly published 1/29/05