Yesterday was the first day of summer, and it was seriously hot (just a little while ago it was chilly and wet). The agapanthuses are coming into full bloom; they have a long blooming season, extending through the warm part of the year here (which is most of the year).
They are old favorites of mine, a symbol of California for me from my first visits here, in the early 60s. (For years we had a big vase of silk agapanthuses in our Palo Alto living room. Eventually, they got tatty, and last year I replaced them with two vases of intense anemones and Japanese irises.)
But only yesterday did I get curious about the name.
First, something about the plants. The OED2 entry:
A genus of South African liliaceous plants, having large umbels of blue, violet or white flowers on a stout scape, of which the bright blue and white species are commonly cultivated for ornament; also, a plant of this genus. [cites from 1789 on; the year 1789 is important]
They have long strap-like leaves as well as those noticeable but not obtrusive flowers. Excellent for filling spaces and serving as backdrops. Some blue agapanthus flowers:
and a border of mixed blue and white plants:
[Digression: Umbel-bearing plants abound, not all in the family Umbelliferae, but also elsewhere. Wikipedia on umbels:
An umbel is an inflorescence which consists of a number of short flower stalks (called pedicels) which are equal in length and spread from a common point, somewhat like umbrella ribs.
Umbels are a characteristic of plants such as carrot, parsley, dill, and fennel in the family Apiaceae; ivy, aralia and fatsia in the family Araliaceae; and onion (Allium) in the family Alliaceae.
and on the Umbelliferae:
The Apiaceae or Umbelliferae (both names are allowed by the ICBN) is a family of usually aromatic plants with hollow stems, commonly known as umbellifers. It includes angelica, anise, arracacha, asafoetida, caraway, carrot, celery, centella asiatica, chervil, cicely, coriander/cilantro, cumin, dill, fennel, hemlock, lovage, Queen Anne's Lace, parsley, parsnip, sea holly, the now extinct silphium, and other relatives. It is a large family with about 300 genera and more than 3,000 species. The earlier name Umbelliferae derives from the inflorescence being generally in the form of a compound "umbel", and has the same root as the word "umbrella".]
[Another digression: The taxonomic status of the agapanthuses is something of a morass. From the Sunset Western Garden Book, 2001 (p. 174):
Names of species have been much confused over the years, largely because the plants hybridize so easily. It has even been suggested that all agapanthus are merely forms of one species.
There are both deciduous and evergreen species. The evergreen A. orientalis (A. praecox orientalis) is the most commonly planted.
Wikipedia notes ten Agapanthus species listed by taxonomists, plus A. orientalis. The cultivar A. praecox goes by the names Common Agapanthus, Blue Lily, African Lily, or Lily of the Nile, but in the U.S. it's most usually sold as lily-of-the Nile or lily of the Nile, a name that's geographically way off, since the plant is a South African native. From the Plantz Africa site:
The Agapanthaceae is a monotypic family (consists of only one genus) that is endemic to southern Africa, i.e. Agapanthus occurs naturally nowhere else on Earth. It consists of six variable species that are widespread in all the provinces of South Africa except for the Northern Cape, and in Lesotho and Swaziland but not in Namibia or Botswana. (link)]
Now the name. I had a moment of illumination when I realized that the first part of the species name was surely Greek agape (in English, with accents on the first and last syllables) — one of the Greek ‘love’ words (agape, philia, eros).
[Digression: Agape has lots of meanings and nuances in different contexts. OED2 glosses the Greek etymon as 'brotherly love', but notes the development to the New Testament sense of 'Christian love' (with the Latin correspondent cāritāt- ); see the Wikipedia entry for a confusing tour of the historical uses.
Distinguishing agape and philia (roughly, 'friendship', later 'attraction') is not an easy task historically. Note Philadelphia, the City of Brotherly Love, with its connection to the Society of Friends (the Quakers).]
For agapanthus, OED2 gives the etymology
mod. Lat. < Greek ἀγάπη love + ἄνθος flower [agape + anthos]
but doesn’t explain it. The About Flowers site gives the history, but doesn’t explain the name:
its modern name dates from the publication of [William] Aiton’s “Hortus Kewensis,”  wherein, on the authority of L’Heritier, it is entered by the name now universally recognised. It was cultivated in the Royal Gardens at Hampton Court in 1692, therefore it is no novelty; and yet of its history there is not much to be said.
Finally, the Plantz Africa site ventures a tentative guess:
The name Agapanthus is derived from the Greek agapé love and anthos, flower. There is no clear reason for this derivation although it could be interpreted as ‘lovely flower’ or ‘flower of love’. Agapeo means ‘to be contented with’ which is a possible derivation, i.e. ‘flower with which I am well pleased’.
But no one seems to have any idea what was in Aiton’s mind when he coined the name. Were there cultural associations in the 18th century connecting agapanthuses and brotherly/Christian love or contentedness? Or was Aiton, faced with the need to concoct numbers of taxonomic labels, just on a poetic roll? (Note that agapanthus is a satisfying double trochee.)
Calling for a cultural historian!