Best way to manage this unruly den nobile hybrid

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They are being bred for smaller height, to make them more attractive for windowsill growers. Maybe you have one of those.

Don't take the keikis into account when deciding when to fertilize.

Many Dens. make multiple growths per season. I start fertilizing when the first ones emerge, and stop in August or September. My growing season is longer so I may fertilize later in the calendar than might be appropriate in cooler areas.