Five different energy grass plants (reed canary grass, tall fescue, orchardgrass, tall oatgrass, red top) were identified and studied for the purpose of determining the fuel energy qualities of the plants&rsquo; mass while focusing on ash fusion temperatures. The plants were cultivated on four different locations and harvested in various times of the year (early summer, autumn and spring of the following year). It was found that the ash fusion temperatures of plants harvested in early summer were substantially lower in comparison with the autumn and spring harvest. The analysis of the composition of the ashes gathered from samples of grass plants harvested in early summer contained a substantially higher level of potassium, higher level of sodium and higher level of anions Cl<sup>&ndash;</sup> and PO<sub>4</sub><sup>3&ndash;</sup>. SiO<sub>2</sub> is the most represented component in all of the ashes, with the late harvest having approximately 2&ndash;3 times higher level than the early one.