Model letter (semi-formal)
Dear Mr Collins,
Thank you for your note. I'd be very happy to look after your plants while you are away next month. Gardening is one of my hobbies, so it will be a pleasure rather than a chore, and I'll make sure they are watered every couple of days, as you asked.
There is one small problem, however. I have just found out that I need to attend a family wedding in another city on the weekend of the 14th, so I will be away from Friday evening until Sunday night. I'm worried that the plants on your balcony might dry out during that time, especially if the weather is warm.
I have spoken to my sister, Laura, who will be staying in my flat that weekend, and she has offered to water them on the Saturday. If you are comfortable with this, I could give her your spare key on Friday and collect it again when I return. Alternatively, I could move the balcony plants into the shade before I leave.
Please let me know which option you prefer.
Best regards,
Elena Rossi