This detail comes from Pliny the Elder, who reports it in his Natural history 19.40, near the start of his discussion of silphium (tr. Rackham).

... in the consulship of Gaius Valerius and Marcus Herennius [93 BCE], 30 pounds [pondo xxx] of laserwort [laserpicio, i.e. silphium] was imported to Rome by the government, and that during the dictatorship of Caesar, at the beginning of the civil war [49 BCE] he produced out of the treasury together with gold and silver 1500 lbs. of laserwort plant [laserpicii pondo md].

So not Caesar's own personal stash, but state owned, which Caesar had control of as dictator. This wouldn't have been the plant itself, but the juice, which was preserved for trade and transport by mixing it with bran or beanflour, as Pliny explains just above this passage.