Mark Preston wrote:
The first blooms of the season of my Don Juans have an etherial
fragrance in the morning. But my noon it vanishes. All the following
floweres never have any fragrance.

I live in Southern Calif. The roses get the correct amounts of
fertilizer, light and water. Anybody got the same problem?

Here in semi-coastal Northern California, this is a phenomenon I see
only on our occasional hot days. Perhaps in your SoCal climate, the hot
dry afternoons dry up the volatile essential oils in the blossom and
therefore the fragrance disappears as the day heats up? After all,
fragrance of a rose can only be sensed if the volatile essential oils
are air-borne, but the different oils have different molecular weights
and volatility and some may be lost sooner if there is too much heat or
wind or lack of moisture in the air?

I have been told that a warm, humid, bright and still morning can
produce the best conditions to enjoy the fragrance of a well nourished
and watered fragrant rose, and I do find this to be so around here. We
rarely have much humidity around here, but morning is the most humid
part of any day all the same. Also, mornings tend to be not too windy
and wind always picks up in the late afternoon, and in some seasons it
is more than in others.

Anyway, that would be my hypothesis to explain what you are observing.

--
Radika
California
USDA 9 / Sunset 15