Garden Detective: planting Knockout roses

But be warned: If your soil already is alkaline -- with a pH of 7 or higher -- adding ashes could interfere with your plants' ability to thrive. And if it happens to be too acidic, you can't count on ashes alone to raise the pH substantially.

The only way to know what soil conditions you're dealing with is to test the pH level either with an at-home test kit, which you can find at local garden centers or online retailers, or by bringing a soil sample to your county's Cornell Cooperative Extension office (call 516-565-5265, extension 7 in Nassau; 631-727-4126 in Suffolk). Cornell will offer recommendations for improving your soil based on the test results.

Coal ashes and ashes from charcoal briquettes should never be incorporated into garden soil because they contain toxins.

Many thanks to eagle-eyed readers Jennie Townsend of Center Island, Mario Facinelli of Northport, Vinnie Drzewucki of Freeport, Leo McSherry of Hempstead, Christopher Camastro of Springs, Joan Prior of Port Washington, Kenneth Hoffman, Case Joosse and Patricia Garry, who called me on this, giving me the opportunity to set the record straight. Facinelli sent a photo of his own trifoliate orange.

Now, a bit about Poncirus trifoliata: Also called "hardy orange," the trifoliate orange is a multibranched tree covered in super-sharp, 2- to 4-inch-long spines. White flowers cover the tree in spring and are followed by fruits that turn yellowish-orange in autumn and hang on after the leaves have fallen. The tree thrives best in full sun but can handle a bit of shade. The fruit is not harmful, but is considered inedible because it is extremely tart, although it can be used in highly sweetened jam. Trees also can be grown in a row and kept trimmed as a hedge; their vicious spines should discourage animals, trespassers and robbers.