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We cut our poinsettia back as you suggested some time ago. Now it is about 18 inches tall and quite shapely. What else can be done to make it bloom for Christmas? Poinsettias need to have their daylight hours shortened in order to trigger bud formation and reddening of the bracts that surround the buds. In fact, they need 14 to 15 hours of total darkness from early October till late November.

Cover with a dark cloth or bag or put in a dark room from about 5 p.m. to 7 or 8 a.m. each day so no light hits the plants. The buds take about six weeks to form.

The plants need a good light room with indirect or direct sunlight and a nighttime temperature of about 65 degrees F. The soil should be moderately moist but not soaked.

If you fail to give it all the day-night treatment it requires, it will probably bloom for you in late January.

A few years ago we bought some Colorado blue spruce seedlings, which have grown well although only about one-fifth of them show the real bluish color. The others have varying degrees of blue, but most of them are just greenish. Is there something we can add to the soil to make them more blue? When Colorado blue spruce are grown from seeds instead of cuttings you cannot depend on their being ''true blue.'' Varying degrees of blueness are the result, as you have discovered.

You can, however, sprinkle iron sulfate under the drip of the branches at the rate of 1 pound per 3-foot-diameter spread of the branches.

When buying blue spruce from a nursery, keep in mind that they have their best color in the spring and early summer. Summer and fall rains usually wash off some of the powdery substance, known as ''bloom'' or sheen.

We mulched our roses with about 4 inches of peat moss, thinking it would keep moisture around the roots during the summer. When I noticed the leaves were wilting, I dug under the peat moss and found the soil to be bone-dry, even though it had rained the day before. Can you explain this? Peat moss is a good soil amendment when it is worked into the soil, but it does not make a good mulch.

During dry spells it forms a crust on top, making it completely impervious to water. As the peat moss under the crust dries, it too becomes impervious to water, causing the moisture to roll right off. The peat moss must be stirred up and soaked with water to which liquid detergent has been added (a teaspoon to 2 quarts).

The solution also can be used to moisten dry peat moss in a bale or bag.

We planted some Explorer potato seeds and got about a bushel from a 15-foot row of plants. We are curious as to why a third of them are red and the other rest are white. The tubers are mostly oval-shaped and quite nice. A few are long and narrow. Explorer potato seeds are not hybrids (seeds that are produced by crossing the same two parents each time). The seeds are the result of random pollination, so you are getting a mix of what the various parents are like.

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