As they say in landscaping circles, weeds don't cause an unhealthy lawn... an unhealthy lawn causes weeds. (Photo by SharkyinColo via Flickr.)

Did you know that fertilizing a weed can actually help you get rid of it in your landscape?  While this is not the case for all weeds, it is certainly the case for weeds that thrive in infertile soils (knotweed is an example).  As evidenced by the dozens of different fertilizer formulas at Home Depot or Lowe’s, different plants have different fertility requirements that must be met for that plant to thrive.  As anyone who has “burned” an impatiens or coleus when applying high nitrogen lawn fertilizer knows, if a plant is fertilized outside its tolerant range, it can die.  Just like other plants, weeds have their own fertility requirements and some of them thrive in infertile or improperly fertilized soils.  Getting rid of these weeds could be done by simply fertilizing the soil properly.

In fact, some weeds really only show up once your soil is infertile.  Otherwise, while their seeds may be in your lawn (and there are thousands of weed seeds in your lawn at any given time), they won’t fully develop into mature weeds because they are not in their preferred environment and cannot compete with your lawn for water and nutrients.  Proper lawn cultivation is one of the best things you can do to keep your lawn weed free.  If your lawn is happy, it will have less barren spots for weeds to thrive in and the lawn will crowd out most upcoming weeds.

Why this works: There are various mechanisms behind why changing the fertility of the soil affects some plants and leaves others undisturbed.  I’m going to take the simple route in this explanation and only talk about the main one.  Plants need a certain combination of nutrients and, more importantly, soil acidity to be able to not only absorb, but use nutrients in the chemical reactions that occur within the plant’s cells.  When there are missing elements, lack of water, or the soil acidity is thrown off, the plant is unable to complete some of these reactions and begins to have problems (much in the same way a human suffers from deficiencies in food, water, or important vitamins.  A slight deviation from the plant’s preferred environment (such as when acid-loving azaleas are planted in slightly alkaline soil) will yield a plant that continues to live, but doesn’t thrive.  A greater deviation may have catastrophic results for the plant and it may be unable to absorb nutrients or complete most, if not all, its chemical reactions.  Much the way a human would starve without food, the plant withers and dies.

Of course, different lawns have different fertility requirements and there are a lot of different lawn varieties out there so I will not be going into how to fertilize your lawn correctly on this post.  I will leave that to future lawn-type specific posts, but a trip to your local garden center or a search on the Internet should yield some information in the meantime.  It is important however that you do know the fertilizer requirements of your lawn because just as you can get rid of some weeds simply by the way you fertilize, you can get rid of some lawns by using the wrong fertilizer or applying the product too often.  Some lawns, such as some of the Paspalum varieties used in golf courses and popular with some homeowners in coastal areas, require little fertilizer use.  A word of caution: if you don’t know what type of lawn you have, don’t apply fertilizer or herbicide to it until you find out.  Like I said before, you can seriously harm your grass.

This is a good time to mention that once the soil under your lawn is properly fertilized, you may see some weeds wilt and die (what you see when you someone says they “burned” a plant with fertilizer) and others will just look sickly, die over a long time, and never come back.  However, not all weeds will be affected by this and new weeds may emerge when the newly fertilized soil meets their fertility requirements.  Don’t worry… as I have said before, once your lawn is healthy from being properly fertilized it will have a better chance of competing with weeds.  Here in the southeast, it is my experience that the weeds that show up in infertile soil are much worse than those that show up on properly fertilized soil.

A note to organic gardeners:  As an organic gardener you know that you need to tolerate some weeds in your lawn since most herbicides don’t meet the organic motto (although there are a few ones out there that have varying levels of success).  Of course, you can hand weed the lawn (which I will be talking about next), but that is an extremely time consuming task.  However, using organic fertilizers, you can apply the same principles that you see in this post to an organic lawn.  Since organic fertilizers often have less “kick” to them than the ground water-polluting regular varieties, you may not see an immediate effect in getting rid of weeds.  As your soil’s fertility improves over time, you will definitely see fewer weeds.  A true organic gardener, though, tolerates quite a few weeds in his or her lawn.

Previous post in the series:

• How to get rid of weeds in your lawn. Part 1

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