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Pinching (or "topping") dahlias is an easy but somewhat counter-intuitive way to ensure your dahlias are stronger and more productive than they would be if simply left alone to grow.
We wait to pinch our dahlias are about 8-14" in height and have four sets of leaves on the stalk. We carefully snip off the portion of the stalk above the fourth set of leaves (when counting from the bottom!) and then move on to the next plant.
At this point, the plant is growing rapidly and pinching will encourage the plant to develop new lateral buds and stems. While one main/central sprout was removed, the plant will generate two new ones. And by removing the central sprout (and, eventually, a bud) the plant re-directs its energy into new stems rather than an initial central flower which is often on a disappointingly short stem.
This sacrifice may cost an extra week or two without blooms, but will result in a healthier, more vigorous plant that will yield more flowers over the season.
If you are reluctant to pinch your dahlias, don't worry. They will grow fine if left alone. Over time, you may wish to experiment with multiple plants of the same variety by pinching one and leaving the other alone. That's how we convinced ourselves...
There are several good videos available online - just searching for "how to pinch dahlias" and enjoy!
The best time to harvest dahlias is in the morning before temperatures warm up appreciably, or in the evening just before dark after temperatures have moderated from the afternoon heat.
We use sharp, clean scissors to cut stems at an angle just above a leaf node. Over time we have learned we can "train" plants to develop longer stems by cutting deeply (and often sacrificing small side-buds which would eventually become flowers). We prefer fewer flowers with longer stems, but by "pinching" dahlias early in the season we've all but ensured a bountiful harvest of blooms in the last 10-12 weeks of the growing season.
Most dahlias do not open further once harvested, so only cut flowers that are reasonably or fully open. We immediately (but carefully) strip leaves from the stem and place in fresh water.
Dahlias thrive when regularly picked. Allowing old dahlias to remain on the plant for an extended period will result in flower production declining substantially. We harvest and "deadhead" flowers twice a week almost without exception.