Answer: Usually fungal infection is due to either overly wet soil or overly wet foliage with poor air circulation; using infested cuttings or supplies can also contribute to its occurrence as can overly warm or cool temperatures. Cuttings are also sensitive to the time of year they are taken in terms of their predilection to root quickly. Although hydrangeas are considered easy to root, there are a lot of factors that need to be taken into account.
Summer is the best time to try these cuttings. You want tips that are firm but not woody, so about early June or so. This also coincides with warm temperatures outdoors as well as allows time for good root development before fall planting time.
Using rooting hormone is not strictly necessary; some rooting hormones are stronger than others and may inhibit rooting in some plants as a result, some plants are more sensitive than others to different hormones. Some also contain a fungicide which might be useful in some cases but again is not strictly necessary. Some gardeners routinely use them, some never use them. Be sure to read and follow the label carefully and take care you do not contaminate the remaining powder by dipping into the container itself. In my experience, it is not needed for hydrangeas but you could always experiment and see what works best for you.
First off, I would suggest cleaning your pots and tools and the growing area with a bleach-and water solution, one part bleach and nine parts water. Use a sterile soilless potting mix and freshly moisten a batch specifically for this project. Make it just barely damp like a wrung out sponge, it should definitely not be dripping water when you squeeze it. Take the cuttings from healthy plants and immediately plunge them into the mix, firm it if necessary so there is good contact between the cutting and the mix. Now place the pot in a clear plastic bag but keep the bag from touching the cuttings. (You can use a bent coat hanger in the pot or similar to prop it up and away.) Place the pot in a bright location but out of direct sun, direct sun will cause it to overheat. Now be patient. Open the bag occasionally to allow for air exchange. You should not need to water as the humidity inside the bag is pretty steady. Once it seems they are well rooted, roots should begin within a few weeks, gradually open and then remove the bag allowing them time to acclimate to the open air. Also move them gradually to brighter light or similar to their growing location.
Another alternative is to simply leave the cuttings outside on the north side of the house where it is bright but shaded from direct sun. The typical summer heat and humidity provide good rooting conditions so you do not need the plastic covering. Check periodically to make sure the soil is still moist, water if needed.
I hope this helps you trouble shoot. My first inclination is to guess they are too wet, or that the potting mix is not suited to cuttings and is causing you problems. Look for a soilless mix, without fertilizer in it if possible. Good luck with your cuttings!
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