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Lawns/watering the lawn in hot weather

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Question
QUESTION: hi Ronald!

My local weather is hot and dry; there is no rain.

How do I know whether it's time to water the lawn?

How do I know how much water to apply?

A separate question...I have several rhododendrons near each other. Spread out among them is a heavy concentration of weeds. I want to spray a weed-killer such as Roundup, but I'm concerned that the Roundup fluid will damage/kill the rhododendrons as well.
What can I do the prevent the damage to the rhododendrons?

Thanks for your help

Chris

ANSWER:  Look closely at the grass blades. When the moisture is right, they are open flat - like the pages of a book. When they are under moisture stress, they fold along the midrib; or roll up rather like a newspaper. It is quite normal for the grass blades to show signs of moisture stress during the hottest part of the day. But as the temperature goes down the blades open once more.
Apply enough water to wet the top 6 inches of the soil or until the water starts to run off on to sidewalk or street - whichever occurs first. If runoff occurs before the top 6 inches are wet, then you will need to water more frequently until that condition is attained.
If the weeds are GRASSES, you can apply a product containing Fluazifop e.g. "Fusilade".
At the label rates it will kill emerged grasses and not the listed broad leaf plants. The product has a very comprehensive list of desirable plants which will not be harmed.
If the weeds themselves are broad leaf plants, you can use glyphosate very, very, carefully. Add a dye to the spray mixture and if you accidentally get it on a desirable plant, snip off the sprayed parts - immediately! For tight spots, I have used a paint roller to "wipe" the weeds. But I always have the pruners handy.

---------- FOLLOW-UP ----------

QUESTION: Thanks for your reply.

Another question:
What is the right time to water, when the weather is real hot?
Morning, evening, or what?

thanks again.

Chris

Answer
In the morning is optimum. There is little or no wind, and the sun will soon be up to dry off the grass blades.
The evening is probably the worst timing. The sun goes down. The grass blades remain wet and dewfall will ensure that the grass blades remain wet for 12 hours or more!

I am sure that I do not have to tell you that this is the ideal condition for fungi attack.

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Ronald B Persaud

Expertise

Florida (Central and South) lawns maintenance and troubleshooting.

Experience

Work experience in Garden and Technical Sales (Ace Hardware and Lesco). L&O Spray Technican/Horticulturist/Tree permitting (Local Municipality). Commercial Landscape Maintenance (Condo complex and Shopping Mall)

Organizations
Past member Florida Landscape Maintenance Association (Pinellas County).

Publications
On the 'Gardenweb' under the pseudonym "Ronalawn82"

Education/Credentials
CPO certified (L&O) Florida. Hons. Graduate: Eastern Caribbean Farm Institute (now Eastern Caribbean Institute of Agriculture and Forestry), Trinidad WI.

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