RE: Vic -- Iris virginica --

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Mark,

I usually add ¼ cup of 10-10-10 to the 18 –gallon tubs that contain my irises (including Louisiana type varieties & species, virginica, etc.) except I. tridentata.  This is done usually do this in the fall and occasionally again in the spring.  I usually try to pick a day to add the fertilizer when the water is low in the tubs so that when I add water the fertilizer will be washes into the soil.  Since there is no drainage in the tubs, the fertilizer will not be leached  into the groundwater and will last until used by the plants or volatizes. The nutrient I am most concerned with adding to the soil in the tubs is N and as times I have used other fertilizer formulations with less P & K but adjusting the volume to add approximately the same amount of N.

Vic