The hardness of wood is a critical factor in determining its suitability for various applications. Among Poplar, Spruce, and Cedar, Spruce is generally considered the hardest. This is based on the wood's Janka hardness rating, which measures the force required to embed a .444-inch steel ball halfway into the wood. Spruce typically has a Janka hardness of around 550, while Poplar ranges from 400 to 600, and Cedar falls between 300 and 800. Despite the variability in these ratings, Spruce consistently ranks higher in hardness compared to Poplar and Cedar.

The reason for Spruce's higher hardness lies in its cellular structure. Spruce wood has a denser grain and a higher proportion of latewood, which is the part of the tree that forms later in its growth and is typically harder. This denser wood composition makes Spruce more resistant to wear and tear, making it a preferred choice for structural applications. For instance, Spruce is commonly used in construction, furniture, and musical instruments due to its strength and durability.

Poplar, on the other hand, has a softer grain and a lower density, which contributes to its lower Janka hardness rating. While Poplar is still a versatile wood, it is more prone to dents and scratches and is not as suitable for heavy-duty applications. It is often used for furniture, cabinetry, and interior trim due to its workability and relatively low cost.

Cedar, while softer than Spruce, is harder than Poplar. It has a distinct aromatic scent and is valued for its natural resistance to decay and insects. This makes it a popular choice for outdoor applications such as decks, fences, and shingles. The Janka hardness rating for cedar can vary widely depending on the specific type, but it generally falls within the range of Poplar, making it a middle ground in terms of hardness.

In conclusion, Spruce is the hardest among Poplar, Spruce, and Cedar, primarily due to its denser grain and higher proportion of latewood. This inherent hardness makes Spruce more suitable for demanding applications where strength and durability are crucial.