Aspen Hybrid Pumpkin Seeds. Aspen is a semi-bush variety producing 3 to 5 pumpkins per vine. Spherical fruits are a dark orange colour averaging 8-9 kg (18-20 lbs) and 25 cm (10″) across. Aspen produces a strong, heavy handle and stores quite well. As pumpkin seed does not tolerate cool soil, it is best to wait until the soil warms to at least 25 C (77 F) before sowing.