Planting forest on cutover peatlands may be regarded as a viable restoration technique in western Canada, where natural bogs are treed with a high density of Black Spruce, Picea mariana. Fertilizer is needed to promote P. mariana establishment on cutover peatlands; however, it also encourages spontaneous colonisation by non-peatland species such as Paper Birch, Betula papyrifera. This study aimed to assess the most appropriate fertilizer dose for P. mariana establishment and growth against the trade-off of birch invasion; consequently, we monitored the effect of B. papyrifera on P. mariana growth. Four levels of fertilizer dose were applied below-ground, but flooding of the site following planting allowed fertilizer to reach the surface and favoured the colonisation of B. papyrifera. Seven years after planting, fertilizer promoted P. mariana survival and the highest fertilizer dose improved both P. mariana and B. papyrifera growth, while the lowest fertilizer dose promoted spruce growth, to a lesser degree, without promoting birch growth as much as higher doses of fertilizer. Birch removal had a significant positive effect on the growth of P. mariana, possibly by allowing greater light penetration and higher near-surface soil moisture. Avoiding B. papyrifera colonisation on site is more effective than cutting due to the ability of birch to regenerate rapidly from stumps. In practice, if planting coniferous trees is the chosen restoration option, the risk of birch colonisation can be minimised by leaving a thicker remnant peat deposit, burying fertilizer near the planted seedlings, and planning planting to avoid flooding during the growing season post-planting whenever possible.