For Halloween in Sleepy Hollow, NY, I turn our boring white hydrangea bush into a flourescent nightmare. I spray-paint the large flowers--which are mostly dead by October anyway. In the day, it's just garden-variety weird. I've had people ask what flower it is or what I added to the soil to make it those colors. By night, I put on the black light and it glows.

Supplies

PAINTING

Several cans of flourescent paint. Green shows up best. If you can, get the "marking" paint, which allows you to point the can down.
Painting clothes, including gloves.
Face mask and glasses, for your comfort.
Ladder, for taller branches.

LIGHTING

Any kind of black light built for the outdoors.

Step 1: Draw Your Project, Picking the Dark Side of the Bush

Pick a bush or side of the bush that gets completely dark at night. A street lamp will completely washout the impact of the black light. You may want to place several blacklights around the bush from different sides.

Plan out a place for your light to point completely inward toward your yard. You don't want the light to shine into people's eyes. Preferably, the light fixture itself can't be seen.

Make sure your extension cord reaches the area.

Blacklights tend to have a three-prong grounded plug and may not fit into the end of a light string.

Prune back any shrubbery that gets in the way of the light or the from the street.

Step 2: Paint Your Hydranges

Paint your hydrangea.

Shake spray paint well.

No need for a primer coat on the flowers. If you are painting other items to go along with the hydrangea, they might benefit from a primer coat. For instance, if you've picked up any ironic lawn statuary, you'll want to paint it white first.

Be sure to get the underside of the flowers. When children walk by they're going to see them from underneath. I used to use an elaborate newspaper cone around the flowers, but discovered there was no need. Just do it freehand. Paint will mist and drip, so cover anything underneath you need to protect (like planters or sidewalk).

Vary your colors. Green seems to show up best at night, but orange is the natural choice for Halloween.

I recommend using a mask, especially when you're on a ladder doing the higher flowers. Take breaks. Or even do the painting over several days.

You don't have to paint the whole bush. You may want to only paint the part visible from the street and dark enough to be hit by the light.

You may need a second coat. After a storm, see if you need to do touchups.

Step 3: Add Your Lights

Set up your lights.

Don't be picky. Really any black light that's suited for outdoors will do. I haven't found much of a difference in the performance of the various elements of my black light collection. So I go for as powerful of an LED outdoor stage light as I can afford.

Step 4: Turn on Lights and Enjoy

You'll probably have to adjust your lights a bit. For the most part, you can just set them on the ground. You can also try placing them on buckets and using clamps. You may want to replace your porch light with a black light bulb.