We don't have enough information to be able to truly help you with your problem, but I'll make an educated GUESS that your rosemary has been overtaken with spider mites...which nasturtiums are very prone to. Those parts of the rosemary not growing in the full sunlight needed could really lose the battle against the mites.

Plants must have the sunlight in order to make enough energy through photosynthesis to fend off pest problems, or at least survive them.

Having said that about the mites, you still need to remember that rosemary is a plant that requires full sun in order to thrive. It cannot be happy when covered up with other plants.

I would absolutely cut the rosemary all the way back to the ground and see what happens. I've done this many times to my rosemary and it has never failed to resprout like crazy. Mine haven't been hindered by lack of sunlight, though.

Hi rhizo,

Thanks so much for your advice.
It will certainly be cut back. You've exposed another possible drawback - apple and quince trees are reducing sunlight to that area anyway now, something that hadn't been a problem previously - but as I mentioned, parts not covered by nasturtiums were healthy-looking.
I'll cut it back without fail.

Thanks again!

I wish I had such luck with rosemary that it grew like crazy at any time of year! smile The clay soils and cold wet winters here always get mine! The thick nasturtiums are what smothered the rosemary leaves (since the ones not buried under the nasturtiums were healthy..) Unless your rosemary grows like wild, I'd take care not to cut it all the way back - maybe some selective pruning would work just as well.

rhizo_1, that's good news that you've cut rosemary back to the ground and had it re-sprout like crazy. As I type this, I'm pondering a now-huge rosemary that I've managed to over-Winter twice, and I've been wondering about its long-term prospects. Some of its leaves are discolored, and knowing that it could survive heavy pruning might give me the courage to take aggressive action in the Spring.

I've grown rosemary for ornamental purposes on several properties over the years, and have found it to be very amenable to annual rejuvenation pruning. It begins to look woody and unkempt if not pruned to the ground occasional.

I cut back my rosemary back to about 3" above the ground and I'm hoping I didn't kill it. I have since found out that they are considered evergreens, so I'm very nervous and scared it's dead.