Be sure that your plants are indeed edible parsnips, because there are some weeds, some toxic, that are lookalikes. If you are absolutely sure, then you might just count days to harvest (about 100 to 130 days since you saw them sprouting), and not worry about the frost, even though the roots may not be as sweet. On the other hand, you could try letting a few plants die back naturally, and leave some of the roots in the ground until a few fall frosts hit them and see if the quality is retained. I’m not sure, though, if the warm soils of summer will preserve them. It might be better just to harvest them soon.
Note: You don’t want the plant to flower, which will make the roots woody (these plants are biennial, so if the seeds just sprouted this year, it is unlikely that the plant will flower this year unless stressed). Be careful about handling parsnip, as the sap can be irritating to the skin.
Be sure that your plants are indeed edible parsnips, because there are some weeds, some toxic, that are lookalikes. If you are absolutely sure, then you might just count days to harvest (about 100 to 130 days since you saw them sprouting), and not worry about the frost, even though the roots may not be as sweet. On the other hand, you could try letting a few plants die back naturally, and leave some of the roots in the ground until a few fall frosts hit them and see if the quality is retained. I’m not sure, though, if the warm soils of summer will preserve them. It might be better just to harvest them soon.
Note: You don’t want the plant to flower, which will make the roots woody (these plants are biennial, so if the seeds just sprouted this year, it is unlikely that the plant will flower this year unless stressed). Be careful about handling parsnip, as the sap can be irritating to the skin.
Good luck!