Question
My hydrangeas have bloomed beautifully..but now some or most of the flowers are losing their color and drying out. Should I clip off these flowers or leave them?
The plants are usually in good bloom till fall or later. Will removing the old blooms allow for new ones now? Will it hurt the plant?
Thanx, Donna

Answer
Hydrangea problems? Blame them on global warming.

If you've been suffering through the blood-boiling temperatures of the national heat wave, imagine having to lie out there in the sun from dawn to dusk.  Extreme heat bleaches the leaves of susceptible plants.

Hydrangeas are tough plants, but that usually applies to their ability to withstanding cold and wind.  They bloom most happily, and most beautifully, in moderate temperatures with plenty of sun.

But their flower pigments are vulnerable to several current environmental factors.  One is UV-B light.  The other is excessive temperatures.

Anthocyanins are one of the common color biomolecules you find in flowers.  They start to break when exposed to heat.

Hydrangeas have an optimal temperature range of up to around 85 degrees F.  When temps rise higher, they self-cool.  They're very good at this.  Touch a Hydrangea leaf and it feels cool -- because water inside the leaf is evaporating.  The plant is sweating and like people it keeps it from getting too hot.  It's critical of course to keep a Hydrangea from getting thirsty, because it would stop sweating.  Botanists call this plant sweating 'transpiration'.

The trouble now is the blistering heat wave and the low humidity that comes with it.  In dry air, the leaf cells close their tiny pores ('stomata' to botanists).  This causes heat to build up inside the plant.  It is not long before leaf temps reach fatal levels.  This point of no return is called the 'thermal death threshold'.

As if that were not enough, photosynthesis comes to a halt when you hit 90 degrees.  Chlorophyll simply does not function any longer.  Photosynthesis is a chemical reaction.  You need several enzymes for it to work, and as you may know, enzymes have optimal temperatures.  As temps rise, enzymes work better and better.  Then you reach that threshold temperature, and enzymes become less and less effective, until they don't work at all.

In a heat wave--especially when you have a series of these over several weeks or months--you actually COOK these enzymes.  The technical term for this is 'denatured'. Heat begins destroying enzymes.  Photosynthesis comes to a screeching halt.  The plant just stops making chlorophyll.

Think, Donna, what would happen if we could not breathe when the thermometer hits 90 degrees.  That's what it's like to be a Hydrangea.  They cannot breathe.

As for the flowers, well, they are the collateral damage of all this pain and suffering.  Metabolism rises with temperature.  Their life expectancy shrivels in this weather.

How do you care for a poor plant in this situation?  There are important things to do.

First, keep your shrubs watered.  Let the H20 sink deep into the soil, well under the roots, to extend the moisture AND keep roots as cool as possible.

Mulch to retain water - and again keep roots as cool as possible.

REMOVE flowers as they fade.  This is an exercise that should be done for every plant you own.  Cut at the base of the flower to keep it from pouring all its energy into seed production.  Making seeds is exhausting for a plant.  You don't want them to waste a drop of energy, especially in weather like this and, when the weather becomes civilized, the recovery period that follows.

Finally, Donna, as a point of information, you should know that Hydrangea flowers are built during the previous year.  The flowers you have now were all made last Summer.  Removing flowers that are spent -- or that you want to use for bouquets, which these do so beautifully -- will have no effect on next year's flowers and certainly not on this year's flowers.

What WILL ruin next year's flowers: Pruning.  Be careful with the pruning shears; make sure you know exactly what you're doing.  Any questions, please ask.  Thanks for writing,

THE LONG ISLAND GARDENER