The Dahlia Guide - Growing
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Pinching
About a month after you’ve planted your dahlias you should start having your plants up above the ground. Once your dahlias are 4-6” above the ground or they have 4 leaf sets, pinch off the growing tip. What you’re doing is saying ‘stop focusing on growing the one main bloom and focus on the growth of laterals’. This will result in a reward of more blooms over the season. It is hard to do because it makes you wait a little longer for your first blooms, but it is worth it.
Fertilizing
It’s advised once your plants have been above ground for a month start a fertilizing regime. Every single gardener does this a little differently. That’s because no two gardens are in precisely the same conditions.
The typical advice is to use a low nitrogen fertilizer. If your dahlias have too much nitrogen they focus all their energy on leaf production and not flower. And let’s be honest, the flowers are the best part.
If you’re really serious about getting things right, get a soil test before your season begins and make amendments based on your soil test results. I haven’t gotten that hardcore yet, I just fertilize with a low nitrogen fertilizer like Alaskan Morbloom Fertilizer.