1. Protected areas (PAs) are a key intervention for conserving biodiversity and ecosystem services. A major challenge for PAs is the control of invasive non-native plants that spread into PAs from surrounding sources such as forestry plantations. The links between invasions and different source populations are poorly understood, making it difficult to apportion responsibilities for control costs. 2. We estimated the effect of plantations of invasive pines (Pinus species) on the spread of these trees into PAs in South Africa's Cape Floristic Region (CFR). We assessed the effect of (1) distance from plantation; (2) plantation and PA orientation in relation to the prevailing wind; and (3) the proportionate size of the surrounding plantation on the abundance of invasions in PAs. We also estimated the historic and potential future costs of controlling invasions in the PAs. 3. PA management units within 3 km of a plantation had over double the pine cover of control units >6 km from plantations (6.12% vs. 2.39%). We attributed 51%, of the pine invasions in PAs to plantations. 4. Neither the proportional size of the plantations, nor their orientation relative to prevailing winds had a detectable effect on the cover of invasive pines. 5. Costs of controlling invasive pines in the study area totalled 98 million Rand (10 Rand ~ 1 US$ in 2013) between 2001 and 2012. It could cost between 273 and 916 million Rand (all future costs expressed in 2013 Rand), and take between 34 and 113 years, to clear the remaining invasive pines in the study area, depending on rates of spread and the cost-effectiveness of control. 6. Policy implications. Estimating the contribution of an invasion source, in this case forestry plantations, requires estimating an unobservable counterfactual outcome: the invasions that would have occurred in the absence of the plantations. We have made a first step toward doing this by using empirical approaches that vary in the strictness of their assumptions, along with robustness tests to that assess the plausibility of these assumptions. Our study provides the starting-point for estimating the contribution of plantation forestry to protected area invasions in the Cape Floristic Region of South Africa.