The rising cost of peat and pine bark has boosted the demand for alternative organic materials for container growing media. Here, composts of invasive acacia (Acacia longifolia and Acacia melanoxylon) residues were evaluated as alternative organic materials for horticultural substrates. Compost bulk density was less than 0.4 g cm−3 and total pore space was more than 85 percent of the total volume, as established for an ideal substrate. The matured acacia compost air capacity, easily available water, buffering capacity, and total water-holding capacity were also within acceptable recommended values. With increased composting time the physical characteristics of the composts were improved, but the same was not true for chemical characteristics such as pH and electrical conductivity. The replacement of pine bark compost by acacia compost in a commercial substrate did not negatively affect either lettuce emergence or lettuce growth, suggesting that acacia compost can be successfully used as an alternative component for horticultural substrates.