As a rule of thumb, you can collect around 600 gallons of water per 1000sf of roof, per 1″ of rainfall. Please check out this link for the average rainfall in your area.
Average temperatures and rainfall in US cities.
It is wonderful to go for the largest rain barrel size that would maximize the amount of rainwater collected, however any size you can afford and accommodate is good. Also remember to get a rainwater collection system that is closed, so that plant material and insects don’ t collect inside, and use the collected water within a reasonable amount of time (as needed during the days following the rain), so that it doesn’t get a chance to get stale.
How much and when?
Feeding: Some people like to compare the synthetic fertilizers with drugs. They have high potency and feed only the plant, which becomes dependent, not the soil, which in time deteriorates and does not replenish its resources to allow plants to thrive. If you have to use fertilizer, try an organic, slow release product twice a year, following the manufacturer’s instructions. Don’t fertilize in the fall, except for newly planted bulbs. Plants need to adjust to going dormant and eating less. Otherwise, the best option for fertilizing would be your home made compost, which turns all your plant waste into a rich, nourishing material resembling top soil. Quick tip: you can drop plant material in not so visible locations of your flower beds and they will turn to compost without the pile and the smell, enriching the soil in place.
Watering: Some say that watering is a luxury, not a necessity, and the plants, other than the ones in containers, should be able to thrive on the rainwater available that year. I say if the dirt looks dry, water. There is nothing that looks sadder and more neglected than a garden full of wilted plants.
The summer garden has a more tired look than the exuberant garden of spring. It needs a little more care to look its best:
- make sure to water it often enough (if the plants look wilted or the dirt cracks, it has been too long)
- keep the plants deadheaded and remove the dead leaves from plants that go dormant after the spring bloom
- keep the weeds in check, they will sprout out of nowhere and take over if you let them
- feed the plants for a second wave of bloom
- wait for fall to move perennials, especially the larger clumps, heat can be very stressful for a transplanted plant.