Maturity, which significantly affects the potential use of compost as a peat alternative substrate, or substrate component, is difficult to measure, and laboratory-based testing methods are generally unattractive to farmers, due to cost and time delays. The accuracy of relatively simple, quick, and inexpensive plant-exclusive and -inclusive methods for evaluating maturity of rotary barrel composted banana leaf (BL) and lawn clipping (LC) for use as seedling starter and transplant substrates for production of tomato (Solanum lycopersicum L.) and sweet pepper (Capsicum annum L.) seedlings were investigated. Nine substrates were tested: BL or LC compost mixed at rates of 20%, 50%, 80%, and 100% (by volume) with 0% compost; that is, PRO-MIX ‘BX’/Mycorise PRO (Premier Horticulture Ltd., Dorval, Canada) alone as the control. Compost rate affected substrate electrical conductivity (EC), which, in contrast to seedling emergence, survival, and aerial growth, increased with increasing compost inclusion rate. Compost maturity test methods produced no, low, and high levels of phytotoxicity results. Neither BL nor LC compost consistently had a greater phytotoxic effect; instead, the effect seemed to be crop specific. Leaf area was more sensitive to compounds present in the compost than root growth, and EC was the most important factor inhibiting seedling emergence, survival, and root length of tomato and sweet pepper. Microorganisms indigenous to LC compost likely suppressed the positive stimulatory effects of compost extracts diluted at 1:20 v/v on sweet pepper germination and root elongation. Solvita (Woods End Research Laboratory, Inc., Mt. Vernon, Maine), a rapid (4 h) compost maturity test, in combination with a seedling growth assay may be the simplest, least costly, and most useful method to evaluate compost use potential. PRO-MIX ‘BX’/Mycorise PRO amended with BL or LC compost at 1:5 v/v is a good alternative to peat-based substrates for use as a seedling starter and transplant substrate for production of sweet pepper seedlings.