Our backyard has very low spots where the water from the roofs of two adjacent houses ends up. How do I deal with this? I would like to plant a garden of shrubs and perennials but don't think many can take that much water. I have attached photos. Red maples and birch seem happy, but the hydrangeas I planted last year all died. It gets quite a bit of sun.
Most plants will not tolerate sitting in standing water or saturated, soggy soil for long periods. You may be able to add one to two inches of soil to fill in low spots or raise the grade enough so that water will run off better or at least not accumulate there. A steep grade is not necessary or desirable because in dry years you do want the water to sink into the soil and down to plant roots.
This past year we had abnormal rainfall — about twice the average. Many people lost plants in areas where they had grown for years but were now under water too much for the plants to survive. The maple you have may be red maple, which is happy even in a bog; the birch is probably a river birch. Hydrangeas love moist soil, but cannot tolerate standing water. In saturated soils, the water pushes out the oxygen roots need. Eventually the plant drowns, unless it is a plant adapted to saturated soil, i.e. a bog.
You may have a good location for a rain garden. Many plants love this environment — some stunning natives in particular, such as button bush and clethra. (Both are also butterfly magnets!) Search ‘stormwater-water-raingardens’ on the University of Maryland Extension’s Home and Garden Information Center’s website.
Don't get bogged down (no pun intended!) with details. Just plant what likes "wet feet." Native plants are best. For more plant choices, look at the U.S> Fish & Wildlife Service’s excellent online publication, “Native Plants for Wildlife Habitat and Conservation Landscaping: Chesapeake Bay Watershed." In the lists, 'Plants for Freshwater Wetlands and Other Wet Sites' should be helpful.
You may not have standing water continuously in the future. However, it is predicted that we can expect a lot more wet years and extreme weather ahead because of climate change. A rain garden is a smart way to handle this, as long as this spot does not hold water all summer (and breed mosquitoes). When you install a rain garden, the plant roots will be pulling in the water and drying up the low area, too.
Is dormant oil toxic? How does it work and when would I spray it?
Dormant oil is simply horticultural oil mixed for use in colder weather, usually sprayed in late winter/early spring but also late fall. This relatively inexpensive insecticide breaks down in hours, yet can give you a jump-start on pests such as scale and mites over-wintering in bark fissures or leaf crotches.
Horticultural oils are usually petroleum oil with the plant-harming chemicals removed, plus an emulsifier so they can be mixed with water. (There is some plant-based oil on the market, too.) Horticultural oils work by creating a physical barrier to insect respiration, i.e. clogging the spiracles (breathing holes) along the sides of adult or larvae. Similarly, on insect egg masses the oxygen absorption is blocked and fewer eggs hatch. Insect metabolism decreases as temperatures decrease so, to be effective, oil concentration must be higher in winter than in summer. Thus you find that winter or dormant rates are different from light summer weight oils. Horticultural oil also can be a feeding deterrent, interfere with egg-laying, and act as a toxin to some extent, absorbed by insects and interfering with metabolism. Follow the temperature parameters on the label very carefully.