The lichen is indeed harmless but it needs fairly mature bark to get a toehold. Those stems look very elderly. The shrub could probably do with a good rejuvenation prune.

Forsythia stems don't last long...a 5 year old stem is usually on the decline and by 10 years, probably dead. You should consider pruning out those old stems when they hit 5-6 years old. Alternatively, a more drastic rejuvenation prune as suggested above.

I was going to a hard prune in November. Although there is conflicting advice on the web as to the best time to do it. I would prefer to keep some green in that spot for the summer though. Is it harmful to do a partial hard prune? Maybe just get rid of the two really old branches at the ground????

Thank you
Jill

Hard pruning of these types of early blooming shrubs is best done as soon as possible after flowering to maximize the flowering potential for the following season. But rejuvenation pruning - removal of the oldest growth (up to a third of the total stems) to the ground - can be done pretty much anytime.

Next spring's flowers will be on the new growth made this year - the long whippy branches, so you don't want to cut all the new stuff, just the old branches with not much growth coming out of them. Cutting the really old stuff to the ground is exactly right, so have at it.

bingo .. and with no insult to the root mass .. you will be very surprised how big new growth can be by fall ...

ken

btw.. i was chuckling the other day.. when i noticed i had lichens growing on my plastic mailbox ...

think of them as a bonus show from ma nature...

https://duckduckgo.com/?q=lichens&t=ffcm&iax=1&ia=images

ken

Best time to prune forsythia is right now! Don't wait any longer.