While the benefits of biophilic design, incorporating natural elements to promote well-being, are well-demonstrated in Immersive Virtual Reality (IVR) environments, its application remains underexplored. Specifically, no studies have clarified how biophilic elements should be spatially and contextually integrated into IVR environments. Most studies focus on the presence or type of elements rather than on design strategies or spatial criteria. This gap is critical in IVR, where the absence of real-world constraints enables a high degree of design freedom, potentially leading to ineffective or inauthentic integration. To address this issue, we engaged 13 expert designers in a focus group to develop guidelines for biophilic design in IVR workspaces. Each expert was asked to enrich four virtual offices using four types of indoor plants: floor plant, table plant, hanging plant, green wall. Their insights were synthesized into practical spatial design guidelines. These were then combined to generate 16 biophilic configurations, evaluated by 81 users. Results indicated no statistically significant preference among configurations, suggesting all combinations were similarly received. In a second evaluation phase, 117 participants compared four specific configurations: based on guidelines; with an unconstrained (but realistic) spatial arrangement; with an unrealistic spatial arrangement; without plants. The guidelines-based configuration significantly outperformed the others across subjective measures including willingness to work, focus, and comfort. These findings suggest the proposed guidelines are effective in supporting the design of engaging, pleasant biophilic IVR environments.