UNITED STATES—Superbloom occurs only about once every decade or so. It is very unpredictable though. It can happen for two or even three consecutive Springs. Alternatively, it may not happen for two decades. It can be as early as late February, or as late as late May. It can last one or a few weeks. Superbloom is as variable as the environmental factors that influence it.

Actually, all bloom relies on influence from variable environmental factors. That is how it knows how and when to bloom. It is also why many species are so irregular about doing so. Various species prioritize reliance on various environmental factors. Therefore, some may bloom early or late while others adhere to stricter schedules. It can get complicated.

Warmth is undeniably the most significant of environmental factors that influence bloom. Cool weather inhibits bloom, even as growth resumes at the end of winter. Then, warmth accelerates bloom, mostly during spring, but also into summer. Many species continue to bloom as long as the weather stays warm. Very few prefer to bloom during cool weather.

Environmental influences are . . . complex.

As mentioned, the influence of environmental factors can get complicated. Many species actually require sustained Winter chill, or vernalization, to bloom well. It is how they reset their growth seasons to begin in Winter. Peony is unreliable here because of inadequate chill. After unusually cool winter weather though, flowering cherry and lilac bloom better.

Humidity is another important environmental factor. Although it does not stimulate bloom for more than a few species, it can prolong bloom. Some flowers, particularly from humid climates, can desiccate with aridity. Conversely, even flowers from desert climates retain hydration better with humidity. Also, rain provides water for areas that lack any irrigation.

Species from mild but not tropical climates might rely more on photoperiod than weather. Poinsettia in the wild does not experience sufficient chill to distinguish Winter. It monitors the photoperiod, which is the duration of daylight, instead. It knows to bloom when nights are longer than days. A few species use the same very consistent technique to bloom for other seasons.

Highlight: ‘Little John’ Bottlebrush

Old fashioned lemon bottlebrush would be so practical, if only it did not get so bulky. It is impressively resilient. Although appreciative of occasional watering, it needs none once established. Phases of red bloom continue through warm Spring and Summer weather. A few sporadic blooms might continue throughout Winter. Hummingbirds are very fond of it.

‘Little John’ bottlebrush, Callistemon viminalis ‘Little John’, is all that and less. It remains much more proportionate to compact home gardens. Growth is relatively slow and tame. Mature specimens may grow no bigger than three feet tall and five feet wide. They prefer to assume naturally mounding form without shearing. Any pruning should be quite minor.

More than compact form distinguishes ‘Little John’ bottlebrush from lemon bottlebrush. Its evergreen leaves are relatively small and slightly grayish. Their foliar texture is relatively tidy. Its lemony foliar aroma is relatively subdued and only evident if foliage is disturbed. The fuzzily staminate floral trusses are relatively small with relatively mellowed red color.

Tony Tomeo can be contacted at tonytomeo.com.