<italic>Miscanthus</italic> is a close relative of <italic>Saccharum</italic> and a potentially valuable genetic resource for improving sugarcane. Differences in flowering time within and between <italic>Miscanthus</italic> and <italic>Saccharum</italic> hinders intra- and interspecific hybridizations. A series of greenhouse experiments were conducted over three years to determine how to synchronize flowering time of <italic>Saccharum</italic> and <italic>Miscanthus</italic> genotypes. We found that day length was an important factor influencing when <italic>Miscanthus</italic> and <italic>Saccharum</italic> flowered. Sugarcane could be induced to flower in a central Illinois greenhouse using supplemental lighting to reduce the rate at which days shortened during the autumn and winter to 1 min d^-1, which allowed us to synchronize the flowering of some sugarcane genotypes with <italic>Miscanthus</italic> genotypes primarily from low latitudes. In a complementary growth chamber experiment, we evaluated 33 <italic>Miscanthus</italic> genotypes, including 28 <italic>M</italic>. <italic>sinensis</italic>, 2 <italic>M</italic>. <italic>floridulus</italic>, and 3 <italic>M</italic>. <italic>×giganteus</italic> collected from 20.9° S to 44.9° N for response to three day lengths (10 h, 12.5 h, and 15 h). High latitude-adapted <italic>M</italic>. <italic>sinensis</italic> flowered mainly under 15 h days, but unexpectedly, short days resulted in short, stocky plants that did not flower; in some cases, flag leaves developed under short days but heading did not occur. In contrast, for <italic>M</italic>. <italic>sinensis</italic> and <italic>M</italic>. <italic>floridulus</italic> from low latitudes, shorter day lengths typically resulted in earlier flowering, and for some low latitude genotypes, 15 h days resulted in no flowering. However, the highest ratio of reproductive shoots to total number of culms was typically observed for 12.5 h or 15 h days. Latitude of origin was significantly associated with culm length, and the shorter the days, the stronger the relationship. Nearly all entries achieved maximal culm length under the 15 h treatment, but the nearer to the equator an accession originated, the less of a difference in culm length between the short-day treatments and the 15 h day treatment. Under short days, short culms for high-latitude accessions was achieved by different physiological mechanisms for <italic>M</italic>. <italic>sinensis</italic> genetic groups from the mainland in comparison to those from Japan; for mainland accessions, the mechanism was reduced internode length, whereas for Japanese accessions the phyllochron under short days was greater than under long days. Thus, for <italic>M</italic>. <italic>sinensis</italic>, short days typically hastened floral induction, consistent with the expectations for a facultative short-day plant. However, for high latitude accessions of <italic>M</italic>. <italic>sinensis</italic>, days less than 12.5 h also signaled that plants should prepare for winter by producing many short culms with limited elongation and development; moreover, this response was also epistatic to flowering. Thus, to flower <italic>M</italic>. <italic>sinensis</italic> that originates from high latitudes synchronously with sugarcane, the former needs day lengths &gt;12.5 h (perhaps as high as 15 h), whereas that the latter needs day lengths &lt;12.5 h.