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Scabiosa loves the sun, and is best planted in well-drained (sandy) soil with some organic matter (compost) thrown in for nutrition. 

Water
Plants should be watered well, but not so that the soil becomes soggy. A waterlogged soil during the growing season will weaken them, and kill them altogether during the winter.

Keep those flowers coming
Cutting away spent flowers at the base of the stem will keep 'Grand Stone' producing new blooms all summer. 
This also has the effect of making plants more bushy.

Mildew
Mildew is probably the biggest threat to a Scabiosa's health.
It is best prevented by ensuring good air circulation, so that the plant doesn't find itself trapped in still, humid air.
A plant that is generally growing well will be more resistant to mildew than one which is already weak through insect damage, or over/under watering.
Safer's produce an organic garden sulfur called 'Defender' which is reported to be effective against mildew.

Garden pests
Caterpillars love to munch on Scabiosa leaves, as do greenfly. Check your plants regularly and take the neccesary steps to remove these creatures.