Ask any lawn care professional what the hardest aspect of their lawn care program is, and I’m sure you’ll get the same answer: crabgrass. This annoying weed is extremely virulent, especially in times of drought.  Cool-season grasses like bluegrass and fescues grow well in the spring and fall, but struggle through the summer months. Crabgrass is exactly the opposite, it thrives in hot/dry conditions. Therefore, when bluegrasses and fescues slow down or go dormant during the summer, crabgrass takes off and fills in.

Applying a pre-emergent control is the primary method used to control crabgrass.  The control works by producing a vapor barrier over the soil.  When the seed splits and the tiny grass plant emerges from its center, the barrier prevents its growth.  The control also prevents the growth of desirable grasses, so be sure to let us know if you’re seeding so we can avoid those areas.

 Pre-emergent treatments only work on monocots.  A monocot produces just one leaf at germination.  These are mainly grass plants, but also include purslane, spurge, and a few less common weeds that have the same growth habit as crabgrass.  Broadleaf weeds like dandelion, plantain, clover, and many others are dicots.  Dicots product two leaves at germination and are unaffected by pre-emergent control.  All broad leaf weeds have a main vein running through the center of the leaf with smaller veins branching off of it.  Broad leaf weed controls are applied after germination when the plant is actively growing for best results. 

Attaining a good weed control depends on four factors: product, timing, application, and environment.  We are responsible for providing the right product, at the right time, the right way, but environmental factors can influence the effectiveness of weed control treatments.

In the case of crabgrass control we experienced a particularly bad year in 2001.  The lack of natural precipitation in April and May prevented the break down of pre-emergent granules.  Instead of merging with the soil surface, they lay dormant and degraded through volatilization. One crabgrass plant can produce up to 150,000 seeds.  The overabundance of crabgrass seeds from 2001, combined with a very hot/dry 2002, made crabgrass control difficult once again, consequently in 2003 we had to use more liquid crabgrass control in the spring to provide better soil coverage and better control. Watering in within 7 days also helps.  Mother Nature can make a big difference too. If summer conditions are closer to normal, crabgrass pressure will not be nearly as high this year.


Rain or irrigation within 8-16 hours adversely affects all broad leaf weed control treatments.  A little sprinkle is okay, but a good shower will wash the product off the leaf blades reducing or eliminating weed control.  Unfortunately, the optimum time for treatment coincides with the rainiest time of the year.  A typical spring forecast goes like this:  periods of clouds and sun with a chance of showers, All Week!!  After losing countless hours of valuable work time to faulty forecasts, we determined that we must treat lawns until it begins to rain.  If it’s clearly a wash-out affected customers are put on a list for inspection and re-treatment.  However, if over 8 hours have elapsed since treatment, or showers were not severe, weed control may be fine.  It is therefore important for you, the customer, to monitor the effectiveness of weed control treatments and let us know if remedial work is necessary.  To do this, give the treatment a week to ten days to work, if the weed leaves are curling at this point the control will likely be effective.  If weeds look unaffected, give us a call.  Without your help it will take us six weeks to notice the weed problem making control much more difficult.

Other environmental complications with broad leaf controls are heat and drought stress.  Temperatures over 85 degrees produce heat stress.  A lack of rain or irrigation produces drought stress.  Weed controls are made of salts, which produce chemical stress.  Cumulative stresses can damage the lawn.  Therefore in times of drought, or when temperatures exceed 85 degrees, we cannot apply weed controls to the lawn.  A treatment during a period such as this can “burn” the lawn.  Burning rarely effects the roots but it will kill the top growth.  It will take two weeks of good environmental conditions (daily watering) to re-grow the lawn where the burning takes place.

Few days are appropriate for weed control treatments during July and August.  Crabgrass controls are somewhat less stressful giving us more flexibility with post emergent crabgrass treatment.  Our office staff keeps a list of customers who need extra weed or crabgrass controls during summer months and schedule treatment when conditions permit.  That’s when we see the worst stress of all: customer stress, and personnel stress.  We care deeply for our customers and relish the opportunity to provide the service they need.  Your patience during periods of adverse environmental conditions is greatly appreciated.

Written By: Terence Boots
Massachusetts Certified Arborist
Owner: Emerald Acres Inc.