If the label states you can use as a soil drench, yes. If it is either silent on the topic, or says not to do so, then no.

What kind of soil problem are you trying to resolve?

Hi! I planted a bed of Vincas and they are drooping and also a couple of other potted plants have drooping leaves and its from a lack of water. I sprayed the leaves with Serenade but wondered if I could also do a soil drench.

Have you considered that you might not be watering sufficiently? We can deprive our plants by not watering often enough or long enough. Newly installed plants, in particular, require special attention until they are well established.

Do you feel that the bed of vinca has been under or over watered? By the way, what kind of 'vinca'? Catharanthus roseus or Vinca minor or V. major? I'm guessing the former, since you're in a zone 9 location.

Serenade is a fungicide so spraying plants with it that are drooping due to lack of water with it will do nothing except waste your time, energy, and money. Keep in mind that spraying these poisons around, especially when not needed, can allow the diseases and insect pests to develop immunities to them so they become even less effective over time.
Then again, overwatering can produce the same symptoms in plants.

one should presume that recently purchased transplants come free of disease ...

they are dropping because of the transplant stress ...

to jump to chemicals... is ridiculous.. and most likely.. an added stress ... and a waste of money ...

if a product is NOT labeled as a drench.. then it is not a drench ... and secondly .. unless its a systemic [of which i am not wasting time researching it] ... then applying it to the soil will not be useful ...

were they planted in full sun ... were they hardened off to full sun.. have you dug one up.. to find out if its too dry ... have you noted any indication of a root disease???

ken

In May I had planted this bed of " Cora" Vincas and within 2 weeks 3/4 of the plants started dying with leaves drooping and some turning black. Vincas are prone to fungal diseases as I have had this happen in previous years. Anyway, I pulled all those Vinca's out , then a couple of weeks later replanted new plants. I sprayed the leaves with Serenade at the time of planting in hopes of preventing this from happening again. Now a few days later I am starting to see this happen again with a couple of the plants . That is why I am thinking its something in the soil.

Vinca are subject to root rot and more, especially if they receive excess water.

And overhead watering promotes botrytis, which causes leaves to droop, die and turn black.

The key to effective treatments is to first diagnose the problem.

Need pictures, please

Hi! Here's a picture of the vincas. Was on vacation so didn't post earlier. One by one each plant is being affected by this. Im wondering if I should treat the soil. Thanks for any suggestions