When growing Japanese lace leaf , if that is the kind you are trying to grow, it is better to get seeds, from a parent plant that is laced leaf & located, where there are nearby lace leaved maple varieties that are more likely to produce laceleaf seedlings. Yes, it is better to sow ripe seeds, but lacking access, some may just have to stratify the seeds. Warm soak seeds first, to help rehydrate the dry seeds, then plant in the ground ( unless frozen by now) otherwise place a baggy, with moist coffee filter, then store in the refrigerator. If soil isn't frozen by February, or early March & the seeds haven't sprouted yet, in the refrigerator, then sow in the ground. & they may germinate better, in soil, outside during spring with cool soil & weather. They break dormancy, & sprout rather early in spring.