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Lawns: Winterizers

© Kenneth Joergensen

In the fall, advertisements on TV talk about "Winterizer" fertilizers for the lawn. These products promise to create a greener, thicker lawn in fall, quicker green up next spring, and increased root development to handle winter stress.

What is a "Winterizer" Fertilizer?
Regular lawn fertilizer bags have 3 numbers printed on the front such as 29-3-4. These numbers represent the percentage of each of the 3 major "active ingredients" in the bag:

Nitrogen (N), Phosphorus (P), and Potassium (K);
always listed in this order: N-P-K.

Nitrogen (N), the first number on the bag, is typically the most important element for lawn grasses. Therefore, "regular" lawn fertilizers are often formulated with higher levels of nitrogen compared to phosphorus and potassium, such as NPK 29-3-4 (a common lawn fertilizer grade). In this example, the bag would contain 29% nitrogen, 3% phosphorus, and 4% potassium.

"Winterizer" fertilizers are simply regular turf fertilizer with extra potassium. Typically you will find "Winterizer" NPK formula grades such as 5-20-20 (20% potassium).

Why is Potassium levels higher?
Potassium play very important roles in regulating plant growth, but contrary to common belief, however, potassium is no longer specifically thought to regulate winter hardiness.

Potassium should instead be added year-round to lawns which are deficient in this element. On lawns with adequate levels of potassium, there really is no benefit to using an (expensive) "Winterizer" type fertilizer. Regular lawn fertilizers can be used.

The reason why potassium is still added to "Winterizer" fertilizers today is probably explained by prominent lawn management books which used to advocate its use. Today it is recognized that additional potassium beyond the normal annual requirements of the lawn has no effect on winter performance, although potassium is very important overall. It just does not necessarily need to be added in late fall.

In late fall, the nitrogen is actually more important for cool season grasses, such as fescues, bluegrass and ryegrass. Recent research has even shown that nitrogen is also the most important factor in overall health for warm season lawns. In the past, winter hardiness was thought to be compromised if warm season grass was fertilized too late. This assumption has now been challenged (see GCSAA article, below).

The low nitrogen/high potassium "Winterizer" fertilizers, such as 5-20-20 were mainly devised for warm season lawns and can be contributed to classic text book advice (old habits die hard).

As mentioned above, potassium is in general very important to the health of a lawn, and since potassium also leaches (drain through) the root zone easily, the occational use of a high potassium fertilizer is still recommended. I therefore always use a high potassium fertilizer in fall.

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