Thank you for availing your knowledgable services to others.
My townhouse that I moved into recently has a sloping garden. The growth of annuals and perennials has been poor. Is there anything I can put on top that will retain the moisture.
One of the best ways to help your soil retain moisture is to add a layer of mulch on the surface of the soil, around your plants. Whether it is a popular option such as pine bark or wood chips (preferably not dyed) – mulch helps preserve moisture in the soil. It also helps to suppress the growth of weeds.
But before you do this, you should add a layer of organic material such as compost around your plantings. Soil with insufficient nutrients may be one of the reasons that the growth of your annuals and perennials has been poor. You don’t say whether this is a new townhouse, but it is sometimes the case that in newer developments there is little attention paid to the quality of soil in the garden areas. The best thing you can do to help your plantings thrive is to improve soil quality. Here is a guide produced by the Toronto Master Gardeners in partnership with the City of Toronto: https://www.torontomastergardeners.ca/gardeningguides/improving-your-soil-organically-a-toronto-master-gardeners-guide-2/
Another consideration is the choice of plants for a sloping garden, especially if there is runoff, and considering the amount of sun and shade you receive – another important thing to think about. Here is an excellent article produced by the City of Guelph that might give you some ideas: https://guelph.ca/living/house-and-home/lawn-and-garden/sample-garden-designs/slope-gardens/
You can take steps to improve your soil at any time of year. You should still be able to find composted manure and mulch in garden centres and nurseries.