A white zinnia is rarer than it should be, and 'Polar Bear' is the one worth growing: soft folded layers of creamy white in a full dahlia-flowered form, 4 to 5 inches across, on sturdy upright stems. In a mixed bouquet it gives the eye somewhere to rest. On its own it is simply elegant.
Treat it like any zinnia. Direct sow into warm soil after last frost, pinch the first bud to force branching, and cut all the way back to a leaf node every time you harvest. The white shows mildew spotting sooner than darker colours, so keep the spacing honest and water at the base.
Powdery mildew
White or grey powdery coating on leaves - usually starting on older growth in humid conditions or when nights cool.
Improve air circulation by thinning plants. Apply neem oil or potassium bicarbonate spray at first sign. Avoid overhead watering.
Alternaria blight
Brown circular spots with yellow halos on leaves; spreads rapidly in wet weather.
Remove affected leaves immediately. Water at the base only. A copper-based fungicide can help if caught early.