Believe it or not, such a tiny change makes a big difference. In this case, it's a, but that's an uncommon use. It's not at all incorrect though.
Most people mean to say planted some roses in the (my/our) garden.
That means that this person planted some roses in their garden, or in a specific garden that these particular speakers could mutually refer to as the garden, like, say, a community or church garden for example. It's more likely the former.
When you say planted some rose in a garden, you mean that you just planted flowers in some unspecified, random, or generic garden.
It's not at all incorrect to use the indefinite article as I've done in the above sentence, if that is in fact what you mean; but, as I've said, it's an uncommon usage, especially given the context.
I'd probably revise it. But if you do, you'd have to end the dialog at that point, as the rest would be unnecessary.