To properly take care of a bonsai tree weeping willow (https://abodelook.com/product/willow-bonsai-tree/), the soil should always be kept moist and not waterlogged, such trees love humidity and constant watering, and in warm seasons, the watering should be frequent. Make sure that there is sufficient light (morning sun in summer and partial shade in summer) and the soil is well-draining. Your willow bonsai tree will grow with equal amounts of watering, light and soil.