Actually, he originally planned to let Tangmo cast magic and ignite all the vegetation near the watchtower. However, the problem is that Tangmo's flame-based magic is not advanced, only being at a second or third level. This is not due to her lack of talent, but mainly because she lacked the resources to learn advanced flame magic.
It was now the height of summer, and the air was not dry, so, igniting the green plants surrounding them was quite difficult. The reason why the magic crystal grenades could ignite these plants was because the explosion would release all the magic at once, generating high enough temperature to set the plants on fire. Clearly, it was very difficult to achieve such high temperatures with only spell-based magic.
