Top of pot closeup shows moist potting soil in top center and dry around the edges. As though you are not getting all of it wet when watering. Otherwise if this was growing inside and now its getting 6 hours a day of full outdoor sun you may be cooking the top. In addition the pot is too small in proportion to the top so repotting in fresh mix should be done; it might turn out that relocation into new potting medium might produce an improvement.

i would NEVER put a florist plant otuside in full sun ... too much stress ..and imo.. the florist was wrong for telling you that..

move it immediately to very bright shade ... for at least a few weeks.. harden it off to being outside ..

florist flowers come from greenhouses.. this thing may have never seen direct sun.. until you put it there ..

and too much sun.. and not enough water is not a good situation ...

tip it out of the pot..and take a few pix of the root ball.. and show us thiose.. and just slide it back int he pot ..

your plant is stressed.. fert is not a response.. water properly.. and leave it in bright shade and let it settle down and destress ...

ken

Given the chlorotic appearance of the leaves, I'd say that fertilizer is a very appropriate response. The plant was probably delivered to its POS (point of sale) in spring, and hasn't been fertilized since. That, and the fact that it's seriously rootbound would account for the foliage closest to the trunk being shed. For Hibiscus, a random choice of fertilizer is not appropriate, and you shouldn't even consider using any of the "bloom booster" fertilizers for containerized plants, especially Hibiscus. They all supply much too much phosphorous, and it just so happens that hibiscus likes very little phosphorous compared to the nitrogen supplied. The proper pattern for Hibiscus fertilizers should be a medium amount of nitrogen, a small amount of phosphorous, and a large amount of potassium (K). The NPK numbers should be medium: small: large. If you understand the difference between a fertilizer's NPK %s and its RATIO, a 3:1:5 ratio is about right. If you choose Foliage-Pro 9-3-6, and add an equal measure of ProTeKt 0-0-3 to the fertilizer solution, you're good to go ...... and the FP 9-3-6 can be used as is for all your other plants.

If you winter over the tree, it works best to keep it on the dry side in a cool dark corner of the basement, providing only enough water to prevent the roots from drying out, rather than trying to keep it growing until it uses all of its reserve energy and has nothing left when repot time comes every spring. They should be repotted every spring as their root systems are very aggressive. Repotting is much more involved than potting up - ask if interested in knowing the difference (if you don't already know.

The plant LOVES heat and sun, so start reacclimating it to full sun asap. It's likely you won't fix the bud blast issue until you next repot and get the plant on an appropriate fertilizer.

Bud Blast

What's occurring with the flower buds is called 'bud blast', which can be broadly described as the aborting of buds before or soon after opening. Deficiencies of the immobile nutrients are a common cause. Major changes in the amount of light and water the plant is acclimated to, but over or under-watering can also be causal. Radical temperature, cold air from air conditioners or warm air from heat sources can either cause buds to be aborted, and night to day temperature changes can cause condensation within the bud sheath, which can cause the bud to rot. Chemicals/fumes from gas/diesel engines or ethylene gas from any source can cause bud blast. Aphids and thrips can feed on buds or cause infections that cause them to abort. Humidity too low or plants too close to supplemental lighting are often issues, as is chemical damage related to EC/TDS (salt levels). And sometimes, often infuriatingly, the issue is often idiopathic or remains obscure.
I think directing your prophylactic efforts toward maintaining those cultural practices the plant approves of, particularly soil moisture levels, temperature, and fertility, should be followed. Make sure the fertilizer you're using includes all nutrients essential to normal growth. If you have any suspicions that water/condensation in buds might be an issue, first, don't mist or spray the foliage with water. After the bud erupts, you can try carefully peeling back the bud sheath, which will help prevent water or condensation from causing rot issues. If you find aphids/thrips, treat appropriately.

I can't be more specific w/o seeing the plant. Often, though, in cases like this and given a list of potentialities, the grower is able to eliminate a goodly number of possibilities and concentrate on the more likely of the causes.

Al

Oh yeah, I get the frustration on Google about the over-watering and under-watering. It’s confusing. And a helpful tip still. But for your case, and from what I can see on the photos, the yellowing is that your hibiscus is hurting from too much lighting. Yes, I agree with @ken_adrian Adrian MI cold Z5 on florist plants.

The 6-hours exposure is a problem especially when you live in CA. You can’t blame florist though, such long exposure is okay for those in Zone 9a (Northern CA) and with the assumption that you were heading into winter when he gave you this advice. However, as we can see (I’m replying to this in January, 3 months later), the florist’s advice didn’t work. Anyways, I hope your hibiscus is okay even though I’m late on the thread. I just hope the info will be helpful to someone else experiencing the same problem.

Plants absorb light energy in the form of electromagnetic bundles called photons. Specialized cells (chloroplasts) in leaves contain chlorophyll, a green pigment that plays a major part in photosynthesis and serves as something of a sponge that soaks up the photons. As the load of photons striking the plant increases, so does the rate of photosynthesis, but only up to a point called 'the saturation point' (varies by species), which causes photoinhibition. During photoinhibition no additional photosynthesis can occur and heat energy is transferred to carotiniod pigments and is lost to the surrounding air as a measure to prevent damage to leaf tissues. To a degree, this protective mechanism protects plants from damage to too much light.

At the point where damage DOES occur, the damage is a result of a chemical reaction - photo-oxidation, during which chlorophyll molecules rise to a more excited state than normal. If light levels are high enough, the energy released as electrons in molecules return to their normal energy state may be sufficient to form free oxygen radicals from O2. These are the same O2- radicals found in H2O2 (hydrogen peroxide) and are extremely reactive particles which will readily destroy (oxidize/ bleach) chlorophyll molecules. The entire process and technical term is, as noted, photo oxidation, but we generally refer to it as sunburn.

Photo-oxidation is distinctly recognizable when it affects plants. Affected areas on leaf surfaces usually turn gray or silver (they don't appear chlorotic) as the green pigment, chlorophyll is destroyed. Gradually, the affected area turns tan before desiccation occurs, or in cases where thick succulent leaves are affected, the damage might change from gray/ silver to black before desiccation occurs to turn the affected area crispy.

Leaves can only adjust to an increase or decrease in light intensity to varying degrees, and they are better able to adapt to an increase in light than a decrease. In the case where there is no chemical damage to the affected leaf which is receiving more light than it needs, the leaf will develop an abscission layer which will eventually cause the leaf to turn chlorotic and be shed, but it is not capable of developing this layer within the "couple of days" the plant has been under Dylan's care.

AL