Last year, I watched a couple of plants growing and growing, waiting to see how tall they would get. One was Giant, or Great Ragweed (Ambrosia trifida). This year, I knew what it was, and having a bit of a hayfever allergy, I pulled it out whenever I noticed it. Ragweed, both giant size and regular size, is plentiful around here, however, and I have to admit, I do admire the amazing altitude it achieves in just one growing season, with some plants easily 10 feet tall.
The leaves make the plant easy to recognise long before its height gives it away. Goldenrod gets blamed for causing allergies, but it is actually ragweed that is the problem. Just as a rule of thumb, plants with showy flowers, like goldenrod, have heavy pollen that they rely on insects to carry around for them. Plants with inconspicuous flowers, like ragweed, use the wind to carry their fine pollen grains. Plants that depend on wind-born pollen produce much more pollen to make up for the imprecise nature of this pollination process. More people are affected by ragweed than all the other species of wind-pollinating plants because ragweed is one of the most potent allergens and has the longest season.
The other giant is Wild Lettuce (Lactuca sp.). It is possibly Tall Blue Lettuce (Lactuca biennis), although one guide observes that due to the large amount of variation in this genus, it is often difficult to identify wild lettuce to the species level. It’s more beneign than giant ragweed and has a rather charming fluffy seedhead at the top of that long, long stalk.