It needs to be warm enough that the syrup in the top feeder stays above 45 degrees F. Any time of the day the syrup is below that temperature, the bees cannot use it. Baggies full of syrup can be used earlier if you need because the cluster does a better job of keeping their syrup warm.
Also make sure your temperatures are relatively stable. If it gets really cold at night and then warms up a bunch during the day, the syrup will leak out and drip ice cold misery on the cluster. Like Vance said, a baggie or other feeder that sits in the hive might be a better option.
I have found that top hive feeders attract too many ants and they will run off the bees and tear up the brood. Keep an eye out for this. I prefer the open feeding method because if one hive needs feeding then they all need feeding.
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