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When Temperatures Rise, Don’t Fertilize

If you have cool-season grasses, save your time, money and the Bay

In recent weeks, unscrupulous chemical lawn companies have urged home gardeners to apply fertilizers to lawns during hot, dry weather to keep them healthy and green. Don’t believe them.

If your lawn is primarily bluegrass or fescue, fertilizing is the biggest mistake you can make. The growth of bluegrass and fescue lawn slows when soil temperatures rise into the 80s, and the roots’ ability to absorb nutrients becomes greatly reduced.

Applying fertilizers on hot dry soils is a sure-fire contribution to the pollution of Chesapeake Bay. Cool-season grasses such as bluegrass and fescue absorb most of their nutrients when soils temperatures are in the 60s and 70s. Even if you irrigate your lawn to keep it green during this period of drought, most of the soluble nutrients — such as nitrogen and potassium — leach into soils below the roots zone and, if not absorbed by the roots, enter ground waters. If fertilizer is applied on dry soil and not irrigated into the soil immediately, it will float away and into the Bay during the first heavy downpour.

Applying fertilizers to dry soil is also detrimental to the roots of dormant grasses. Fertilizers are salts. When the concentrations of salts in soil is higher than the concentration of salts in plants’ roots, the soil will pull water out of the roots, killing them. Dead roots soon leads to dead plants.

In dry weather such as this summer, the tops of grasses go dormant, but the roots are still alive. When the rains return and the soils cool in September and October, that is the best time to fertilize cool-season grasses. Next spring, top-dress your lawn with compost, and you’ll find a heartier lawn that looks better throughout drought.

Warm-season grasses, however, such as Zoysia or Bermuda grass, must get a different treatment. If you’re growing these lawns, you must fertilize almost monthly — providing you irrigate frequently. If you have allowed your warm-season grasses to turn brown and go dormant, you should not apply any fertilizers.

Helping Crape Myrtle Flower

Q I have written you in the past about how to get wisteria to flower, and you passed along your expert advice. Now I have another question for you about crape myrtle at the Fairmont Hotel in the heart of the District of Columbia, which my company maintains.

They have a beautiful courtyard there where there is a vast assortment of plants, especially four important crape myrtle installed two years ago. The problem is they are not flowering. We cut them back in late February and have been careful not to put a lot of nitrogen on them. I believe crape myrtles need more sunlight than the three to maybe four hours they are getting.

–Dave Lindoerfer. Silver Spring: Owner Inside Out Services, LLC

 

A For crape myrtle to flower well, they need a minimum of eight hours of direct sunlight. As yours are very much in the open, they are also getting a lot of indirect sunlight. They appear to have put on a lot of growth, probably from having been pruned too severely, perhaps back to stubs. Next season try pruning by removing only branches smaller than a pencil in diameter.

Ask Dr. Gouin your questions at [email protected]. All questions will appear in Bay Weekly. Please include your name and address.

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