Pretty much any mains-powered zigbee Device will act as a repeater. So a lot of this is going to depend on just how bad your weather gets.
Many people have used a zigbee lightbulb in a sheltered location. There’s the problem of dropped messages, but If it’s in a spot with very little traffic then that should be OK. I would try a Sylvania Lightify bulb for this. 
If the weather is really bad, SmartenIT does make outdoor – rated relays for control of pool and pump equipment, but they are very expensive.
There are also a couple of different zigbee irrigation systems where the mains – powered controller would act as a repeater. Again, expensive, Unless you also have a use for the system. SmartenIt makes one and Spruce makes a popular one and I think there are a couple of others.
If you’re up to some technical work you can get a zigbee pro device rated for outdoors and use it for very long distances. See the following thread:
Of course a second Gardenspot should also do it. Remember that in order to get the garden spot which is further away to use the first one (or any new Device) as a repeater, you need to do a zigbee heal after you move things around.
Leave all your zigbee devices on power. Take the hub off of power, including removing any batteries. Leave the hub off of power for at least 15 minutes. This will cause the other devices to go into “panic mode” because they can’t find the hub. Then when you put the hub back on power each individual device will rebuild its neighbor tables, so it will find any new repeaters in the area. This process can take a while, so you may not see improvements in communications until the next day.
In your situation, I would probably just start experimenting with your second Gardenspot, remembering to do the heal after you move it. If you can’t establish communications that way, none of the other devices are going to work either except maybe the pro option which will have a longer range.