The hardest plants to grow indoors are usually large plants or plants that do most of their growth underground, such as carrots and other root vegetables. These plants need space, depth, and conditions that are difficult to recreate indoors.

Plants that grow very tall or wide, such as corn, large tomato varieties, or climbing vegetables, are also challenging indoors. They require strong light, vertical space, and airflow that most homes and countertop systems cannot provide.

In smart gardens, the hardest plants are those with deep roots or heavy fruiting needs. Smart gardens are designed for compact plants like herbs and leafy greens. Root crops such as carrots or plants that become very large often fail to develop properly because the growing space is too shallow or restricted.

In everyday indoor growing, plants are hardest to grow when their natural growth pattern does not match the limits of indoor space, light, and container size.