Walnut Creek’s Ruth Bancroft is a national authority on drought-resistant gardening. Twice a month, she and her staff share their knowledge with readers.
Q Can you tell me the identity of the stemless Aloe blooming at the Ruth Bancroft Garden, with tall pointed spikes of yellow flowers?
A This is Aloe vacillans, native to Yemen and Saudi Arabia. It flowers in November and December.
Q After seeing the silk floss tree in flower at the Ruth Bancroft Garden, I wondered at what time of year it flowers in its native South American habitat — fall or spring? When a plant is moved from one hemisphere to the other, does the flowering switch seasons or continue at the same time as in its natural habitat?
A If the Ruth Bancroft Garden’s silk floss tree were moved to South America, it would flower there at the same season as it does here in the Bay Area — fall. And this is true in general for plants from the Southern Hemisphere: If they are moved far enough to experience a significant change in day length between summer and winter, they will key on day length as the cue about when to flower.
There’s a lot of misinformation on this point, and we sometimes hear it said that the Aloes that bloom here in the Bay Area during winter are doing so because they come from South Africa, where the seasons are reversed — that they flower in our winter because that’s when it is summer in South Africa.
This is nonsense. The species that flower in winter here are in flower six months later in South Africa, during that country’s winter. The plant doesn’t know what hemisphere it is in — it simply responds to the shortening days and cooler temperatures of winter as the cues that its flowering time has arrived.
If a plant is moved from one hemisphere to the other, it is initially thrown off by the reversal of seasons, but soon adjusts to the new situation, synchronizing its growth and flowering according to the seasonal rhythms of its new home.
As for plants from equatorial regions where there is no difference between summer and winter temperatures, and there are 12 hours of daylight and 12 of darkness each day, they respond to weather cues after they’ve been moved elsewhere. These include arrival of the rainy season, changes in barometric pressure, etc. In some cases they settle into flowering consistently at one time of year. But in other cases the flowering here is somewhat random, with no fixed schedule.
Email questions on drought-resistant plants to [email protected].