Yes, you can grow a small lilac in an outdoor plant box, but it requires careful planning and proper care. Lilacs thrive in well-draining soil and need at least 6 hours of sunlight daily. Choose a dwarf or compact variety, such as 'Miss Kim' or 'Tinkerbelle,' which are better suited for containers. Ensure the plant box is large enough (at least 18-24 inches deep) to accommodate the root system. Regular watering, occasional fertilizing, and pruning after blooming will help your lilac flourish. With the right conditions, your outdoor plant box can become a fragrant and colorful lilac showcase.