Learn Russian: Спасайте сад как можно быстрее, а то растения начнут погибать. - Save the garden as quickly as possible, otherwise the plants will start to die.

The phrase как можно literally means "how it is possible." When placed directly before a comparative adverb or adjective like быстрее (faster), it acts as a maximizer. Together, they literally mean "as it is possible faster," which is the standard Russian way to express "as fast/quickly as possible."

Why use the imperfective imperative спасайте instead of the perfective спасите for a single action?

While the perfective imperative is usually standard for a single, complete request, using the imperfective спасайте adds a strong sense of immediate urgency. It translates the feeling of "Start saving it right this second!" In emergencies, the imperfective imperative demands instant action without delay.

Is а то just a colloquial way of saying "otherwise"?

Yes, exactly. А то is extremely common in everyday Russian to mean "otherwise" or "or else." You could use иначе for a more formal or written tone, but а то fits perfectly with the spoken, urgent tone of this warning.

Why must we use the imperfective погибать (to die) after начнут?