How to Use crib rail in garden?

You could use it to hang planters on, as a trellis, or a small place for vines to climb

They make great trellises for vining plants.

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You could use some upside down pots or concrete blocks on either end and make a plant shelf. Or weave some rope through the slats and make a decorative backdrop for potted plants.

Use it for vines!

turn it on its side so the rungs are horizontal and use as a trellis for flowers, etc

A crib rail is an item made to be used indoors, not outdoors. The wood will not have been treated to withstand rain, wind, sun, and general outdoor changes in air temperature. If you do use it for plants and such, just a caution to not spend too much money on the project, as it probably will not last more than one season outdoors. It will disintegrate, fall apart, and likely rot very easily.

In the U.K. drying racks that either hang from the ceiling, or hinge from the wall are largely used to dry laundry. Depending on what type of space you have to work with in your laundry area, I would suggest this would be a more longer time reuse for a crib rail.