Hi, 

You may need to resort to a systemic herbicide. Digging hopefully helped knock it down some, but spraying may take a few applications. If you can dig in some areas, then watch for regrowth (its inevitbale that there will be some pop up still with all those small bulbils). 

Glyphosate or tryclopir are 2 commonly used herbicides that will come in a variety of brand names. They work by translocation through the plant material. If it is lower than 50 degrees, the plants most likely will be in a state of dormancy, so the chemical will not work as well since the movement will be slowed. 50 or above is correct. For lesser celandine (LC) you could try to apply if the night temperatures are in the 40s, below that, and it most likely wouldn't have any effect. LC is a spring plant that will go dormant once it gets too hot outside. It may just take a little longer to see die back in cooler temperatures. Make sure to use a surfactant or "spreader-sticker" mixed in to help the chemicals break through the waxy coating on the leaves.

If the label says AVOID spraying in lower temperatures, that is more subjective and you can test it on cooler days. If the label says DO NOT spray below 50 than that is the law and you should not spray it.

This article from Iowa State Extension explains the temperature range in more detail. 

Let us know if you have further questions.