Stinging Nettle Tips Turning Black

The tips of the leaves on my stinging nettles are turning black. Is this due to excessive watering? Should I replace them and are plants still available for this spring?

Stinging nettles do best in moist, rich, slightly acid soil. If you have allowed the soil to remain soggy wet for very long the roots might have been damaged and as a result the leaves can't get enough water and the parts farthest from the stem start to die off. A more likely reason for root death would be leaving the plant too dry for too long or having a soil pH that is very acid or very alkaline. Wait a while and see if the plants gradually recover as they grow new roots.
If they plants are too damaged to survive, you can reorder some more any time until shipping season ends -usually in October.