“Is this a plant or a weed?” – Fairly common question. Confusing for a botanist because of course weeds are plants.
That’s not a weed; it has pretty flowers! – Some weeds do have pretty flowers. Sigh.
Grasses don’t have flowers – Of course grasses have flowers. They’re just reduced and hard to see.
Everything with long narrow leaves is a grass. – Maybe, but maybe not. Sedges and rushes also have long, narrow leaves and inconspicuous leaves. A long list of plants, some of them dicots, are called “grass.”
Every white-flowered member of Apiaceae (the Carrot Family) should be feared as Poison Hemlock (Conium maculatum). Sigh. Most Apiaceae are not Poison Hemlock and Poison Hemlock itself won’t reach out and poison you. Just don’t eat it. And no, your toddler isn’t likely to stuff his mouth with this – it smells (and presumably tastes) bad.
“Leaflets three, let it be.” – Well, OK, it’s a good first rule if you’re clueless about plants because Toxicodendron (Poison Ivy, Poison Oak) causes such a bad rash. However, many other plants, including strawberries (Fragaria), many raspberries & relatives (Rubus), and Box Elder (a maple, Acer negundo) have three leaflets. They’re harmless.
This has oak leaves – it must be Poison Oak. – No, if it has undivided (but lobed) leaves, it’s an oak (Quercus). Poison Oak (Toxicodendron diversilobum) is a different species in an entirely different family.
Hay Fever is caused by Goldenrods (Solidago). – No, Goldenrods put up conspicuous yellow flowers to attract bees at the same season that Ragweeds (Ambrosia), Kochia (now Bassia scoparia), and many other late-season, wind-pollinated species release their pollen in the air.
Roses cause rose fever, a spring version of hay fever. – No, but they bloom at the same time as many wind-pollinated trees that have inconspicuous flowers and release their pollen in the air.