By the way, and this is partly to defuse the little uncomfortable exchange
on this thread - scientists seem to have established that the date lilacs
bloom in a given location is predictable to a precise degree - based on
degree days above 40 - or in some versions, degree days above 33.
Here is a typical description:
"Bloom time data on the Hort. Farm lilacs was collected in 1978 -79. Bloom
period and degree-day accumulation to bloom was presented in UVM
Agricultural Experiment Station Research Report 1, 1980, entitled "Flowering
and Fruiting of Woody Ornamental Plants in Vermont." To calculate the degree
day accumulation before bloom we used the accumulated average daily
temperatures above the base temperature of 40°F until bloom. For example, a
day with maximum temperature of 60° and minimum of 38° would result in
average temperature of 49° giving a degree day accumulation of 9 degree
days. The earliest blooming lilac cultivars -- 'Evangeline', 'Minnehaha' and
'Lucie Baltet' commenced bloom May 10 in 1979 with a degree day accumulation
of 350. The latest to bloom were Preston hybrids 'Nocturne', 'Miss Canada'
and 'Elinor' which commenced flowering on June 5 with 911 degree days
accumulated. Using the degree day data, one can predict the date of bloom
for various locations for each cultivar for which we have data. "