“shrub, any woody plant that has several stems, none dominant, and is usually less than 3 m (10 feet) tall. When much-branched and dense, it may be called a bush. Intermediate between shrubs and trees are arborescences, or treelike shrubs, from 3 to 6 m tall. Trees are generally defined as woody plants more than 6 m tall, having a dominant stem, or trunk, and a definite crown shape. These distinctions are not reliable, however, for there are some shrubs, such as lilacs and honeysuckles, that under especially favourable environmental conditions, grow to the size of an aborescence or even a small tree. Some specimens of a plant species may take a tree form, whereas others, under different conditions, may assume a shrub or aborescent form, e.g., sumacs, willows, and spruces.”

Bromeliads, cacti, orchids, and some wetland plants are exempt from Growth Zone rules.

It doesn’t matter how tall they are.

All are evening Growth Zone 3 foods (6:30 p.m.-1:00 a.m.).

They march to a different drummer, collecting their carbon dioxide at night and storing it as malate via crassulacean acid metabolism (CAM).

Ulrich Luttge (“Ecophysiology of Crassulacean Acid Metabolism (CAM),” Annals of Botany, Volume 93, Issue 6, 2004) wrote …

“The simplest definition of CAM, first described for species of the family Crassulaceae, is that there is (1) nocturnal uptake of CO2 via open stomata, fixation by phosphoenolpyruvate carboxylase (PEPC) and vacuolar storage of CO2 in the form of organic acids, mainly malic acid (phase I sensu Osmund 1978), and (2) daytime remobilization of vacuolar organic acids, decarboxylation and refixation plus assimilation of CO2 behind closed stomata in the Calvin-cycle (phase III). Between these two phases there are transitions when stomata remain open for CO2 uptake for a short time during the very early light period (phase II) and reopen again during the late night period for CO2 uptake with direct assimilation to carbohydrate when vacuolar organic acid is exhausted (phase IV).”