Killing plants with kindness

Plants are hardier than most of us think.  The shrubs, trees, perennials, grasses, and even annuals of today have evolved to this point by Darwinian means, and their often tender appearance can be deceiving.  Take, for instance, the humble sedum.  The "Autumn Joy," pictured above, is thriving, producing plenty of flower buds in the height of the summer heat.  The sedum's thick leaves are waxy and succulent, storing an abundance of water to be judiciously doled out to stems and roots during even the driest spells.  So if they do well without water, why not hedge our horticultural bets, and shoot a spritz from the garden hose their way every day? 

Bad idea.  Overwatering a plant that is already adept at storing it results in this (pictured below):

Yellowed leaves that drop off when a stiff breeze hits the plant.  What's worse, gardeners think that perhaps the extreme summer temps are to blame because leaves are yellowing: a sign that the plant lacks water.  True.  But yellowing and browning leaves can also be due to too much water, so err on the side of neglect and keep the hose nozzle pointed away from your sedum for a week or two.

Another problem I've noticed this summer in particular is the black spotting of Rudbeckia foliage. 
Spotted leaves on the Rudbeckia, (AKA black eyed Susan) pop up where fungal spores have overwintered and conditions are right for reinfection in the spring. Tight spacing, overhead watering and high humidity contribute to the spread of leaf spot diseases, and the very nature of these plants makes breaking the disease cycle difficult.  Fortunately, the disease doesn't interfere with blooming.  Thinning out plants should ultimately correct the problem.

To maintain proper spacing for good air circulation, actively pull volunteer seedlings that spring from the many seeds Rudbeckia produce in the fall.  Transplant them to other areas of your yard.  In the meantime, removing spent foliage will help in small plantings, since it removes spore sources, but this can be impractical due to the nature of these spreading prairie plants.

Since the spots are mainly cosmetic, this may be a wasted effort if you don’t mind spotty foliage. Many gardeners simply arrange their black eyed Susans in group plantings so the leaves are less obvious as the summer progresses.