Phototropism

Poison-oak seed germinates in moist, lightly shaded locations. 20% of full sunlight is best. The stem then begins seeking the strongest immediately available light. Growth-inhibitory hormones are produced on the side of the stem that is most strongly illuminated, so the stem grows toward the brightest light. The bending occurs smoothly, like this:

Sometimes the environment is uniformly moderately shaded, for example at 5% of full sun, or too dry. Under these circumstances, the main stem runs horizontally. Every 6 to 22 inches a slender side branch with a single leaf is sent up to check out the environment. This growth pattern is called “rhizomatous”. The drawn specimen is from the forest floor along the Oak View trail in the Los Osos Oaks Reserve:

On the next page, when the stem senses stronger light, it turns in that direction. Growth accelerates, up to three feet in a year. This second pattern is called “procumbent”. The drawn specimen grew on the western edge of the abandoned field in the back of Los Osos Oak Reserve; it was removed for public safety:

Because procumbent poison-oak grows so much faster than most of the plants that shade it and is woody-stemmed enough to stand free, it often grows right through the plants that shade it, what Gillis called the “scandent” pattern. In persistently moist situations it reaches heights of nine feet:

However, with support poison-oak grows 75 feet high (Jepson, p. 136)! In southern New England, where I grew up, the most common large vine was wild grape; in Morro Bay, it is poison-oak. If you want to see this, there are fine specimens growing on the sycamores in Toro Creek.

In situations where poison-oak is in the trees; the canopy is uniformly dense, as in Coast Live Oaks; and there is space between the canopy and the ground on the north or east side of the tree, poison-oak branches often hang down from the bottom of the canopy. You can see this at the start of the entrance trail for the Oak Reserve; look for the vines first. This is called the “pendant” growth pattern: