Christopher Baker
Autumn colors emerge when trees no longer produce chlorophyll.

Dear Martha: What causes leaves to turn red and yellow in autumn?

Answer: Actually, the colors are always there. They're just masked by the green chlorophyll in leaves, which is busy making food by photosynthesis while the sun shines.

Come autumn, shorter days and cooler temperatures cause trees to switch into energy-storage mode, at which point their leaves stop producing chlorophyll. For the few weeks before the leaves fall to the ground, they are colored only by their natural pigments. It's these colors — red and purple anthocyanins, yellow and orange carotenoids — that make fall foliage in four-season climates so glorious.

Of course, some years the show is more dramatic than others. The best conditions for intense leaf color to develop are dry, sunny days followed by cool (but not freezing) nights. A warm, wet autumn will almost surely result in less-than-spectacular foliage because the process of chlorophyll loss will be less consistent. Freezing temperatures, meanwhile, can cause leaves to drop suddenly, denying plants the opportunity to enter their slow, colorful dormancy.

Finally, trees that are under stress — whether because of pests, disease, injury or drought — may drop their leaves with no color change at all.

Dear Martha: As winter approaches, how can I ensure that my fireplace will be safe throughout the season?

Answer: As comforting as a fire is on a cold winter day, fireplace mishaps damage more than 10,000 homes each year in the United States. To help prevent such an accident from occurring, have your fireplace inspected at least once a year, as recommended by the National Fire Prevention Association.

Depending on the results of the inspection, your fireplace may need to be cleaned, too. A certified chimney-service professional (a "chimney sweep") will clear your chimney of animal nests and other obstructions, as well as creosote, a combustible substance that builds up inside a fireplace over time. The chimney sweep will also examine the firebox and flue for cracks, which may let carbon monoxide seep into the house.

All told, these services will cost a few hundred dollars (more if extensive cleaning and repairs are required). To find a chimney sweep in your area, contact the Chimney Safety Institute of America at

Once your fireplace is in working order, burn only seasoned wood, and employ a sturdy screen to prevent embers from flying into the room. Never burn treated paper, including the kind used to wrap gifts, as it can release harmful fumes into the home.

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