On the evening of the 12th of June, all arrangements being nearly complete, Banks ordered a vigorous bombardment to be begun the next morning. Punctually at a quarter past eleven on the morning of the 13th, every gun and mortar of the army and navy that could be brought to bear upon the defences of Port Hudson opened fire, and for a full hour kept up a furious cannonade, limited only by the endurance of the Union guns and gunners, for the Confederates hardly ventured to reply, save at first feebly. When the bombardment was at its fiercest, more than one shell in a second could be seen to fall and explode within the narrow circuit of the defences visible from the headquarters on the field. The defenders had three heavy guns dismounted during the day, yet suffered little loss in men, for long before this nearly the whole garrison had accustomed themselves to take refuge in their caves and "gopher-holes" at the first sound of Union cannon, and to await its cessation as a signal to return to their posts at the parapet. They were not always so fortunate, however, for more than once it happened that three or four men were killed by the bursting of a single shell.
When the hour was up the cannonade ended as suddenly as it began, and profound silence followed close on the intolerable din. Then Banks sent a flag of truce summoning the garrison to surrender in these words: "Respect for the usages of war and a desire to avoid unnecessary sacrifice of life, impose on me the necessity of formally demanding the surrender of the garrison at Port Hudson. I am not unconscious, in making this demand, that the garrison is capable of continuing a vigorous and gallant defence. The events that have transpired during the pending investment exhibit in the commander and garrison a spirit of constancy and courage that, in a different cause, would be universally regarded as heroism. But I know the extremities to which they are reduced. . . . I desire to avoid unnecessary slaughter, and I therefore demand the immediate surrender of the garrison, subject to such conditions only as are imposed by the usages of civilized warfare." To this Gardner replied: "My duty requires me to defend this position, and therefore I decline to surrender."