Strelitzia alba and Strelitzia nicolai are similar in hardiness, exposure, poisonous if ingested, bloom color, bloom time, foilage, watering and propagation methods.
Difference is in bloom time, alba blooms in late winter/early spring whereas nicolai blooms in late spring/early summer.

About that 'seed-mutant', I don't know about that, that takes a little digging into my botany books.

Marcus

Thanks for your response, Marcus. The bloom time is a good criterion, but it becomes a problem in my area, as it warms up in December & many plants get confused & bloom early. I have had one that has been blooming since last December & is still blooming. I will watch the bloom time closely, however & watch for a pattern.

Palgrave, 'Trees of Southern Africa', gives the following key to the tall strelitzias. I have condensed it slightly:

Flower heads compound; several flowers in the spathe, then a second complete spathe (with flowers) arising out of the first, a third arising out of the second [and so on, up to a fifth] producing a 'multi-storey' structure; petals blue (very rarely white, with deeply sagittate lobes. S. nicolai. Eastern Cape through Natal and Zululand into Mocambique to the eastern mountains of Rhodesia.

Flower heads simple; several flowers arising from a single spathe; petals blue wih distinct sagittate lobes. S. caudata. Transvaal, north and east Swaziland.

Flower heads simple; several flowers arising from a single spathe; petals white, not blue, and lobes rounded, not sagittate. S. alba. Cape, confined to a small area from George to Humansdorp.

Note that country and province names have mostly changed since this was written circa 1977.

Thanks, tonyfromoz, I have printed out your information & will look at the plants. I will go to my friends house & check out his bird of paradise bloom time  his are protected - like a microclimate - & they may be more predictable as far as bloom time. Do either of you know of a good book strictly on these plants or plants in this family?

I finally figured out my plants  they are Strelitzia nicolai. I went to my friends house, who has the parent plants in a more predictable location. Just like marcusbotanicus said they flower spring & summer. The flowers do form a 'multi-storey' structure as tonyfromoz said. Thanks guys  this is material not found on the net & is very important in differentiating the nicolai from the S. alba.