University of Vermont
Department of Plant and Soil Science

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THE AFTER EFFECTS OF DROUGHT

Dr. Leonard Perry, Horticulture Professor Emeritus
University of Vermont

Most parts of Vermont and, in fact, much of the Northeast experienced a drier than usual summer this year.  This affected many perennials, trees, and shrubs.   What can you expect now, and in the coming year, as a result?  What can you do to help these stressed plants?

If you have very dry sites, like sandy soil and medians near pavement, or new plantings, you already may have lost some plants.  There's not much you can do here except replant and help plants get established in future years. Using organic mulches is a good way to retain soil moisture for the future.

Don't be too hasty to replace plants that appear to have dried up.  Plants that appear to be dead may actually have living tissue underneath the bark, or in the ground.  Scratch the bark of trees or shrubs to see if it is still green underneath.  It's best to wait, if you can, until next spring and see if these plants leaf out. The same applies to woody plants that are living but may appear to have "dead" branches.  Again, use the fingernail test to see if these still have some life.  If so, wait until spring to prune. 

For perennials, prune off obviously dead growth and branches. If the whole plant appears to be dead, mark it to remember its location, as it may produce new shoots next spring.  Browning on the leaves may not be aesthetically pleasing, but leave them as they are still helping the plant. Keep perennials weeded, as weeds rob them of soil moisture.  “Deadhead,” or remove, spent flowers from perennials so the plants will conserve energy from not forming seeds.

Whether you have a sandy or heavier clay soil, top dressing with compost will help.  Generally, the more compost the better.  Organic matter is key to soil health and helping soils to retain more moisture.  This also will help lawns that may have suffered or died during a drought.

Speaking of lawns, you may wish to rent an aerator or get some aerator blades for a mini-tiller to help heavy, baked soils.  For weak or stressed lawns on clay, as well as on lighter sandy soils, you might want to lightly overseed grasses in early fall prior to topdressing lightly with compost.  If seeding, make sure you can keep lawns watered until the new seeds germinate and begin to establish. Maybe it’s time to consider whether some of your lawn can be replaced with easier-to-maintain groundcovers.

Keep all plants watered as well as possible.  This means a good soaking.  Light watering fosters shallow roots, which are quite susceptible to drought.  If you have only a few perennials or shrubs, watering by hand or a slow trickle from the hose may work.  If you can’t water all your landscape if it has been stressed from drought, focus on new plantings, and on trees and shrubs.  Annuals and perennials are more easily replaced if they succumb to drought.

For a whole perennial bed, soaker hoses often are the best method of delivery.  These are porous rubber hoses that allow water to soak right into the root area and not on foliage.  These don't foster leaf diseases, and they don't waste water to evaporation and areas without plants, as do overhead sprinklers.  If using overhead sprinklers, water early in the day to allow foliage to dry before night.
   
From late September into October, it is especially important to keep rhododendrons and other evergreens well- watered.  This will help them get through winter with a minimum browning of leaves.  If they’ve been stressed from too little water during summer, this fall watering is even more important.
   
The usual rule-of-thumb for watering is an inch of water per week, if not from rain then from your efforts.  Get a rain gauge if you don’t have one so you’re not fooled.  What may appear to be a rainy period, in reality may not end up delivering sufficient rain.  Keep in mind that when looking at climate numbers, it is the amount and frequency of rain during a growing season that is important, not the yearly total (which may have come in just one or a few events).
   
Don't fertilize woody plants in early fall as this may promote non-hardy growth, and in late fall it does little good since plants have gone dormant.  However, many herbaceous perennials will respond to fall fertilizer (an organic, slow release form works well) by going into the winter hardier and with more food reserves for the following year.
   
Just keep in mind, too, that what happens one year with woody plants, such as this year's drought stress, often shows up the following year or even for several years after.  You may see plants with less vigor, increasing dieback such as from winter injury, or more susceptibility to diseases and pests. Deciduous trees (those that lose their leaves over winter) which turn color in fall, such as maples, may turn color much sooner if drought stressed. It may take several years of proper care and moisture for plants to fully recover from a very dry summer.
   
Many woody and herbaceous perennials that bloom early in the season set their buds the previous year.  These include lilacs, forsythia, peonies, and many daylilies.  Even the later bloomers may have less growth next year as a result of the stresses this year.  So, keep an eye on these and, if they are not at their best this coming year, don't despair but have patience!
   
If, in any given year, your plants don't bloom or perform well, ask yourself what happened last year.  Were there stresses?  Or, did the plants bloom quite well and now are taking a year to recoup?  Some fruit trees do this naturally and regularly, a natural process termed “alternate bearing.”  Understanding what happened the year before will help you provide proper care--and extra help if needed--for your plants this year. 
     

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