An entire pharmacy lurks outside our doors. That's because flowers, shrubs and trees carry an array of compounds both beneficial and poisonous. Some are even lethal. How many British murder mysteries have you read where the nagging wife or brutish husband was done in by a pinch of aconitum slipped in a salad or cup of tea? It's hard to believe that all parts of perennial monkshood (Aconitum napellus) could be so lethal, even the velvety blue and purple hooded flowers that are such a fine substitute for the harder-to-cultivate delphinium. I grow monkshood in tall spires among hydrangeas, but, despite their beauty, rarely cut them for arrangements, feeling a bit leery of their presence on the dinner table.

Answers to questions about what plants are how poisonous are as murky as the ponds where toxic skunk cabbage grows. A plant can be poisonous for some people but not for others, and in some but not all circumstances. A plant's toxicity varies with the season, age and its genetic composition. A person's susceptibility depends on what else they've eaten, weight, ethnicity and individual sensitivities. Reaction to any compound can be intensified by heat and light — what might not bother you on a chilly, overcast day can cause a rash in hot, bright weather. None of the books will risk declaring a plant perfectly safe. Rather, you'll find they report that in 1956 a cow died in France when ingesting a large amount of a certain plant. I exaggerate, but still, a clear answer is tough to find.

Now In Bloom Acer pensylvanicum is a small deciduous tree that prefers shade and has broadly lobed leaves that turn bright yellow in autumn. The commonly called Moosewood maple is even more striking after the leaves fall; the bark on its trunk and spreading limbs is striped green and white, making it a tree of year-round virtues.

Hence, using caution and gaining knowledge seem good ideas. Giant hogweed (Heracleum mantegazzianum) is a biennial cultivated for its towering umbrellas of tiny white flowers. Its sap can cause terrible skin lesions in some people at some times, and all its parts cause severe discomfort if ingested. I've learned to wear gloves when cutting any kind of euphorbia, for its milky sap contains resins that can cause a nasty rash. It is these same resinous compounds that give poison ivy its name and its itch. And such sensitivities increase over time, resulting in people who are sure they're immune to poison ivy ending up in emergency rooms.

About 10 percent of the calls to poison-control centers are about plants, and most concern young kids who've eaten a bit of a houseplant, the worst ones being dumbcane (Dieffenbachia maculata) and philodendron (P. scandens). With outdoor plants, most often symptoms occur not from poisonous compounds within the plant but the poisons we ourselves have applied — fertilizers, herbicides and pesticides. We worry about kids being attracted to poisonous berries, while people continue to spray perfectly safe plants with poisonous chemicals. Go figure. The rest of the calls are often from adults fallen ill after foraging for wild edible plants. Fatalities occur most often from eating — I bet you can guess — deathcap mushrooms (Amanita phalloides).

If you have questions about the toxicity of wild and garden plants, you might take a look at "The AMA Handbook of Poisonous and Injurious Plants" (American Medical Association, 1985), or "Common Poisonous Plants and Mushrooms of North America," by Nancy J. Turner and Adam F. Szczawinski (Timber Press, 1991).

Humans aren't the only ones who suffer plant poisonings; kittens and puppies are harmed, too. So even if you don't expect to graze in your garden, it's a good idea to be aware of which plants are seriously poisonous to humans and pets. The list includes, to name just a very few, such familiar flowers as daffodils, sweet peas, nicotiana, tulips, chrysanthemum and lily-of-the valley.

Valerie Easton is a horticultural librarian who writes about plants and gardens for Pacific Northwest magazine. She is the co-author of "Artists in Their Gardens" (Sasquatch Books). Her e-mail address is vjeaston@aol.com. Julie Notarianni is a Seattle Times news artist.