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A substrate component (WholeTree) made from loblolly pine (Pinus taeda L.) was evaluated along with starter fertilizer rate in the production of greenhouse-grown petunia (Petunia ×hybrida Vilm. ‘Dreams Purple’) and marigold (Tagetes patula L. ‘Hero Spry’). Loblolly pine from a 12-year-old plantation were harvested at ground level, chipped, and further processed through a hammer mill to pass a 0.64-cm screen. The resulting WholeTree (WT) substrate was used alone or combined with 20% (WTP2) or 50% (WTP5) (by volume) Canadian sphagnum peatmoss and compared with an industry standard peat-lite (PL) mix of 8 peatmoss : 1 vermiculite : 1 perlite (by volume). Substrates were amended with 1.78 kg·m−3 dolomitic lime, 0.59 kg·m−3 gypsum [CaSO4-2(H2O)], 0.44 kg·m−3 Micromax, 1.78 kg·m−3 16N–2.6P–9.9K (3- to 4-month release), and 1.78 kg·m−3 16N–2.6P–10.8K (5- to 6-month release). A 7N–1.3P–8.3K starter fertilizer (SF) was added to each substrate at 0.0, 1.19, 2.37, or 3.56 kg·m−3. Container capacity (CC) was greatest for PL and decreased as the percentage of peatmoss in the substrate decreased with WT having 35% less CC than PL. Conversely, air space (AS) was greatest for the WT and decreased as percentage of peatmoss increased with PL containing 33% less AS than WT. In general, petunia dry weight was greatest for any substrate containing peatmoss with a SF rate of 2.37 kg·m−3 or greater. The exception was that petunia grown in WT at 3.56 kg·m−3 SF had similar dry weight as all other treatments. Marigold dry weight was similar for all substrates where at least 2.37 kg·m−3 SF was used.