To start, you can talk with the upstairs tenant. Standard rental and lease agreements may specifically require a tenant to carry out certain basic activities, such as lawn mowing and weeding. Ask if they are, in fact, responsible for yard maintenance. A gentle reminder might just do the trick.
If talking to your neighbour doesn’t help, write a letter to your landlord. (You can use the Tenant Resource and Advisory Centre’s template letter to help you.) You can tell them how untidy or unsightly the rental property is, and how it's affecting you (including a few pictures can really drive the point home).
Your landlord must investigate your complaint. If they’re aware of the problem and don’t take reasonable steps to protect your right to quiet enjoyment, you can take your landlord to dispute resolution through the Residential Tenancy Branch. If you prove your claim, you could get monetary compensation.