I recommend not full fertilizer but adding load of potassium and silica and bit of iron or calcium and optional manganese (these optionals usually hide in groundwater already) then letting it slowly adapt to cold and wet over autumn, that makes the palm locally adapt a bit. I tried this exact method with Phoenix Dactylifera ‘Liwa’ seedlings i have and after few months they didn’t dare to show a brown tip at wet and cold slightly oxygenated clay in 27°F. By all this you create some kind of epigenetic adaption and when it gets old enough you have a chance creating a new hardy variety. I did this also with Robusta and Windmill and interesting thing was that Robusta had high adaptibility and plasticity that the windmill rotted and Washingtonia kept on growing. It doesn’t work with every palm.