The maximum growth rate occurs in
Exponential phase
Exponential phase or log phase is characterized by a rapid growth in population, which continues till enough food is available.
During Stationary phase rate of growth slows down.
Senescent phase is the final stage of development during which the plant recycles the valuable cellular building blocks that have been deposited in the leaves and other parts of the plant during growth.
Lag phase is the slow and initial phase of plant growth.