I should mention that this is in Sunset Zone 15. The grevillea gets full sun and a good amount of heat in the summer. In winter (like, now) there are some night close to and below freezing, but that seems not to be the issue, as the yellowing started much earlier in the year.

You mention that it is in a half of a wine barrel and did well the first year. You did not mention what mix you planted it in, but I would guess it no longer drains as well as your plant requires. Without enough oxygen in the soil, due to the collapsing of the mix, your plant is unable to take up needed moisture and minerals. My suggestion is a complete replacement of the mix. Use a well drained bark based mix for longest life. Al

Thanks Al! I had planted in mostly cactus/palm mix (commercially bought.) I haven't seen bark based mix offered but will look. What would you suggest as the best time to do this? Right now the grevillea has flower buds, in spite of the yellowing, although fewer than last year, it seems. Stefanie

I would wait to replace the mix until after the flowering is over. Your plant right now is putting all its energy into the reproductive cycle. It might help at this time if you can install a wick up through the drainage hole to improve the drainage now. You would probably have to lay the half barrel over on its side to insert a wick of about a quarter inch diameter up about a foot into the mix. This will only work if the container is elevated some off the ground so the wick will extend a few inches out of the drain. Read the container gardening forum for more information on wicking. When you are ready to repot check with the landscape supply companies where you live. I have a choice of 10 different mixes, available in any quantity, most bark based in Santa Rosa. Al

The secret is to use Iron Chelate. I had a Grevillea Superb that had too much yellow, and my neighbor (who was once president of the university arboretum) recommended the Iron Chelate. That turned it green very fast.