Yes, cast iron outdoor benches absolutely hold up in the rain—especially the heavy, traditionally cast ones you're picturing for a memorial garden. Cast iron is naturally dense and rust-resistant compared to many metals, but it's not rust-proof. The key is the finish and how you maintain it.
Most classic cast iron benches come with a factory-applied primer and either a powder-coated or oil-based enamel paint. That coating is what does the heavy lifting in wet weather. Rain itself won't cause immediate damage; the problem is standing water, trapped moisture in joints, and scratches that expose bare metal. If you get a bench with proper drainage holes in the seat and avoid letting leaves or soil pile up in crevices, it will shrug off years of rainfall.
For a memorial garden, I'd recommend looking for benches labeled “outdoor cast iron” or “commercial grade.” These usually have extra coatings and stainless steel or zinc-plated hardware. A quick seasonal wipe-down with soapy water and a touch-up pen or rust-inhibiting spray paint for any chips will keep it looking classic for decades. You can also apply a clear automotive wax once a year to seal the surface against moisture.
One practical note: cast iron gets very cold and very hot with the weather, so if the bench will sit in direct rain followed by intense sun, expect the wood or cast iron slats to expand and contract slightly. That's normal—just check and tighten bolts annually. If your memorial garden is near saltwater or uses heavy de-icing salts nearby, you'll need to be more vigilant, as salt accelerates corrosion.
Bottom line: a quality cast iron bench is a beautiful, durable choice for a memorial garden, even in rainy climates. Choose a reputable manufacturer, keep it clean, and touch up any scratches promptly. Then all you have to worry about is choosing the perfect spot to sit and remember.