What are the immediate symptoms of too much water?
Surprisingly, plants may wilt even though the mulch or soil surface is wet. Roots need oxygen to transport water so they wilt in saturated soils.
Nitrogen (responsible for leaf color, vigorous growth) is highly mobile in soil and may be leached by heavy rain. Nitrogen deficient leaves will be pale throughout, including veins. New leaves of water-stressed plants may be smaller than normal. Older leaves will show yellow first. It’s possible that even lawns may start looking lighter-than-normal green.
Water-induced nutrient deficiency
Unusual colors that might otherwise be interesting may start showing at the edges or throughout the leaves between the veins. Yews, a landscape staple, are poster plants for wet soil damage. Their needles turn rusty-orange – all the way to the branch tips. Also, keep your eyes peeled for black, purple or discolored bands (cankered) stems on deciduous trees and shrubs.
Is there a difference between wet and flooded?
Absolutely. Areas that often have standing surface water for a day or more require flood tolerant plants (or a great landscape firm to change the drainage!). For example, trees such as Bald Cypress, Swamp White Oak and River Birch (have you seen the cool dwarf cultivars like ‘Fox Valley’?) are perfectly happy with periodic flooding. While it seems logical please understand plants are not Wet-Dry vacs that will pump an area dry. Select flood-tolerant plants for these sites.
'Fox Valley' River Birch
What about containers?
Plants in containers with holes in the bottom drain better than plants in the ground. Most potting mixes are “soilless”, being coarse peat or bark-based, to promote drainage. The downside is minimal nutrient value. So, be sure to fertilize throughout the season (more frequently in sun, less for plants in shade) in years with heavy rain. Be sure that containers with saucers (attached or otherwise) are emptied religiously. After 30 minutes the soil will have absorbed all the moisture it can hold. After that, roots die.
What about insects and disease?
Pestilence is most certainly in our immediate garden future. The jury is out on how this moisture will affect Japanese beetles. Slugs should be rampant. Protect hostas, annuals and other perennials whose foliage rests on the ground. Earth-friendly baits, like Sluggo, contain iron phosphate and are safe even around edibles. Slugs eat the bait, get a tummy ache and crawl away to die in a few days. It’s alright. Chances are, you’ll never run across the proverbial slug graveyard.
Fungal diseases will abound. The usual suspects, black spot on roses, tomato blights, powdery mildew and a jillion leaf spots are a sure bet as we move into heat and humidity. Remember, fungicides must be applied ahead of the infection. They will not reverse symptoms that are already present.
Can I forego watering new plants?
Sure can’t. Rainfall is not cumulative. Do regular checks of newly installed trees and shrubs even after measurable rainfall. Water appropriately. Don't forget to overrride your irrigation system on days it's raining heavily and for a few days after substantial rainfall. Your plants will thank you!!!