Clarification: Sorry. I did not mean to suggest that your plant blooms on both old and new wood. Rather, I was pointing out that there are varieties of Big Leaf Hydrangeas that bloom only on old wood and some that bloom on both old and new wood. The second ones are called remontant cultivars (or "rebloomers" in advertising) and they produce a second flush of blooms in late summer/fall 2023 from new stems that started growing in the spring 2023 or as soon as those new stems get tall enough to produce flower buds and immediately bloom. Your 23 year old plant could be either type as rebloomers appeared in the market around then. You would have to observe if it blooms from new spring 2023 stems this year to see if it is remontant but I will assume it is not. If unsure which stem originated from spring 2023 then monitor starting in 2024.
Review the following general scenarios to see which may apply to the case of your 23 year old specimen and any other Big Leaf Hydrangeas that do not bloom:
* If you prune the ends of the stems AFTER the stems develop dormant flower buds inside the ends of the stems from July-September but BEFORE the stem blooms in spring, there would be no blooms in spring from the stem that you pruned.
* If you let the soil dry out after they develops dormant flower buds, the flower buds may be aborted by the shrub. Always keep 3-4" of mulch and maintain the soil evenly moist in the summer. Reduce watering during winter but do not let the soil get bone dry.
* Depleted nutrients: try determining if there are nutrient deficiencies in the soil by either doing a soil assay or testing the soil with a home test kit that checks the levels of nitrogen, phosphorus and potassium. These kits are sold at some nurseries or big stores like Lowe's/etc. Contact your agriculture extension service for a soil assay.
* Patchy frost damage could be a problem in Los Angeles as it occurs at temperatures as high as 42°F if the dew point and winds are favorable. While a regular frost would basically kill all the flower buds, patchy frost tends to affect parts of the shrub (the top only; or the left side; or the right side; or the front side; or the back side (it varies), etc.
* Old shrubs may start to lose vigor when they get large so for large shrubs, consider also rejuvenation pruning. During the time that the shrub is dormant in winter and for three consecutive years, prune as follows: on year 1, prune the 1/3 longest stems of the shrub all the way down and leave the remaining stems unpruned; on year 2, prune the next 1/3 longest stems of the shrub all the way down and leave the remaining stems unpruned; on year 3, prune last 1/3 longest stems of the shrub all the way down and leave the remaining stems unpruned;
* Fertilize only with a general purpose, slow release fertilizer with a NPK Ratio of around 10-10-10. Avoid high nitrogen fertilizers like some formulations of Miracle-Gro and make sure that lawn fertilizer does not fall anywhere near the hydrangea's root system. Stop fertilizing 3 months before the shrub goes dormant but restart in February if there is no frost or patchy frost.
* Ensure there is sufficient morning sun as a lack of sunlight can reduce vigor and flower bud set. However, this usually occurs when the plants are in dense shade.
