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Kepler K2 long cadence data are used to study white light flares in a sampleof 45 L dwarfs. We identified 11 flares on 9 L dwarfs with equivalent durationsof (1.3 - 198) hr and total (UV/optical/IR) energies of $\geq$0.9 $\times$10$^{32}$ erg. Two superflares with energies of $>$10$^{33}$ erg were detectedon an L5 dwarf: this is the coolest object so far on which flares have beenidentified. The larger superflare on this L5 dwarf has an energy of 4.6$\times$10$^{34}$ ergs and an amplitude of $>$300 times the photospheric level: so far,this is the largest amplitude flare detected by the $Kepler/K2$ mission. Thenext coolest star on which we identified a flare was an L2 dwarf: 2MASSJ08585891+1804463. Combining the energies of all the flares which we haveidentified on 9 L dwarfs with the total observation time which was dedicated by$Kepler$ to all 45 L dwarfs, we construct a composite flare frequencydistribution (FFD). The FFD slope is quite shallow (-0.51$\pm$0.17), consistentwith earlier results reported by Paudel et al. (2018) for one particular L0dwarf, for which the FFD slope was found to be -0.34. Using the composite FFD,we predict that, in early and mid-L dwarfs, a superflare of energy 10$^{33}$erg occurs every 2.4 years and a superflare of energy 10$^{34}$ erg occursevery 7.9 years. Analysis of our L dwarf flares suggests that magnetic fieldsof $\geq$0.13-1.3 kG are present on the stellar surface: such fields couldsuppress Type II radio bursts.

$K2$ Ultracool Dwarfs Survey. VI. White light superflares observed on an L5 dwarf and flare rates of L dwarfs