A question that comes up quite frequently this time of year...what happens if it rains after I fertilize? Is it good ot bad? Will it wash out? Should I re-apply my fertilizer?
The answer not very cut and dry, but here is the bottom line: The vast majority of the time, rain after a fertilizer application is not a problem at all. In fact, it is usually a good thing.
Keep in mind (I have said this before), your lawn needs about 1/2" of water after you fertilizer. Water helps "activate" your fertilizer. It helps move the granules deep into the thatch where it starts to break down so that it can be soaked into the root system. So rain after a fertilizer is a GOOD thing.
The issues is when we get too much rain over a short period of time. The 1/2" mentioned above is not a maximum amount, it can be more than that, but it must be spread over a longer period of time.
There are too many factors at play to say exactly how much rain over what time frame is too much...slopes in your lawn, soil compaction, drainage ditches, how thick your turf is...all of those can play a factor in the amount of wash out you can have.
If you have applied a fertilizer and rain in the forecast is a concern to you, there is something you can do...run your sprinkler system!
That might sound crazy, and this is the only time you should ever run your sprinklers before a rain. However, as I mentioned above, water helps move the granules down deep into the thatch where it is much harder for them to be carried away. Granules also get sticky as they break down which will help hold them in place.
So to recap...rain is good most of the time and should not be a concern. But if it is a concern...run your sprinklers!