I have a potential client that wants to have fresh hydrangea placed between each tier of the cake as shown here:

I primarily work with sugar flowers and use 2" & 3" solid styro foam dummies to space my tiers out. When doing sugar flowers we stick them in the foam filling out the space between each tier for this effect. I have never used tall pillars to float the cake before as I have never had a design that needed them. I'm a bit paranoid about stacking a tall cake on just four pillars with the plate system, another reason we use the dummies and sugar flowers. To me it's just more stable, we can travel with it built, and we finish the cake off with out own flowers, but this is not the case. 

I am to understand that fresh hydrangea need to be watered in order to stay fresh and nice looking during the reception. I also understand that parts of the hydrangea are poisonous, so we can not have any exposed parts sticking in or touching the cake. 

I have no idea how to work with fresh hydrangea, or any other fresh flowers that need to be watered for this concept. Has anyone done this look before? 

I am doing a similar cake here soon. It will be 3 tiers on plates, and in between will be a ring with water and floral foam that the flowers will stick into to stay watered.

AI've done tons of these. They seem tiny, but those supports are strong. They really are. I place the hydrangeas so the stem is on top of the plate. It never really touches the cake.

I should mention I live in HOT Charleston. I usually deliver & set up about 1.5 hours before reception. The flowers will be fine without water. The only one they wilt too quickly is if its outdoors in July or old, bad quality hydrangeas.

Thanks for the replies!

I'm still a little worried about the extra time it will take to setup on site, in addition to the fact that this would be my first pillar plate setup. How long can hydrangea last without water?