Spring showers and summer storms can be welcome sights when you’ve been dealing with heat and drought. While it’s easy to take these natural phenomena for granted, most lawns and gardens wouldn’t exist without the occasional rainstorm!
Unfortunately, a rainy forecast is not always good news. Rain can throw off your mowing schedule just as easily as it can negate the need to turn on the sprinklers. But a surprising number of people don’t realize the impact rain can have on fertilization.
Several factors go into whether you should fertilize before or after rain for the best results. For the average homeowner, however, the answer is surprisingly simple. I’ll break down everything you need to know below.
Should You Fertilize Before or After Rain
I always recommend fertilizing after rain rather than before. While there’s no need to panic if unexpected rainfall hits your garden after you’ve applied fertilizer — you may, however, need to re-apply the fertilizer — you should avoid the scenario if at all possible. Read on to understand all the reasons why.
Why Apply A Fertilizer?
Some people view fertilizer as food for plants but I find that description to be a bit misleading. Instead, it’s more accurate to think of fertilizer as a multivitamin.
Fertilizer is not always necessary for healthy plants — just think of how many trees, shrubs, and even weeds are growing in the world without any help from humankind! But it is a wonderful way to supplement essential nutrients that your garden soil naturally lacks.
Fertilizer is a particularly good investment when growing fruit and vegetables, or any other harvestable crop. Plants that receive the exact type and amount of nutrients they need to thrive grow larger put out more flowers, and produce more fruit than their unfed counterparts.
Most fertilizers contain nitrogen, phosphorus, and potassium. These three nutrients, also known as macronutrients, are the most important for overall plant health. If a plant doesn’t get enough of one or more of these nutrients, it is likely to suffer or even die.
Many fertilizers also include micronutrients like zinc, magnesium, and calcium. While these nutrients are still incredibly beneficial, they play a much smaller role (and are needed in much lesser quantities) than the macronutrients mentioned above.
Nitrogen has several functions but the most important is by far its role in chlorophyll production. Without adequate nitrogen, plants aren’t able to photosynthesize properly. Nitrogen is also crucial for overall growth, so plants that tend to grow quickly will need more nitrogen than others.
Phosphorus is also essential to photosynthesis. Other key functions involving phosphorus include reproduction, root growth, and overwintering.
Plants need potassium to complete photosynthesis and transport water, nutrients, and energy throughout their structures. It also supports healthy flower and fruit production, so is often applied at higher rates to food crops.
Granular vs Liquid Fertilizer
The most popular forms of fertilizer are granules and liquids. Both types of fertilizer provide the same nutritional benefits when used correctly. But there are several reasons you may want to choose one over the other for your garden.
Liquid fertilizer can be a good choice if you have a day or two before expected rain. When applied to either the soil or your plants’ leaves, liquid fertilizer is absorbed more or less immediately.
I do not, however, recommend applying liquid fertilizers if there is any chance of heavy rain within the next 24 hours. There is a very good chance any fertilizer you apply at this time will be washed away before it can be fully absorbed.
If your area is seeing lots of rain in the forecast, granular fertilizer is likely the better investment. This is because granular fertilizer is activated by water — i.e., rainfall — and that is how the nutrients are released into the soil. In fact, most fertilizer granules must be watered in immediately after application.
Since heavy rain can still wash away granules before they’ve fully dissolved, I recommend working the fertilizer into the first few inches of soil. This added layer of protection will minimize fertilizer run-off.
The Effects of Rain On Fertilizers
Rain is a fairly unpredictable factor, and proper fertilization is already hard enough without Mother Nature’s interference. Some of the most common effects rain can have on fertilizer include:
- Diluting liquid formulas
- Prematurely breaking down slow-release granular formulas
- Washing fertilizer off of the foliage
- Flushing fertilizer deeper into the soil
- Carrying fertilizer across the soil’s surface to a different location
I know we’ve focused almost exclusively on rain’s negative impact on fertilizer use. However, there are ways to use rain to your advantage when fertilizing your lawn and garden.
Natural rainfall can provide much-needed moisture to the soil. While heavy rain can saturate the soil — in which case, you should wait a few days before fertilizing — light to moderate rain prepares the soil for amendment without the use of a hose or sprinkler.
I don’t recommend fertilizing when there has been little to no rain recently. Drought puts an incredible amount of stress on turf grass and other plants. Fertilizing during this time is more likely to cause damage than to do any good.
It’s also important to keep in mind that rain affects different types of fertilizer in varying ways. Being familiar with the exact product you’re using will go a long way in ensuring you’re applying fertilizer efficiently and safely when rain is in the forecast.
Watering-In Granular Fertilizer
Granular fertilizers actually benefit from light rain. Granules need water to activate and release nutrients into the soil. At the same time, excess rain could dissolve the granules too quickly and carry them to a different area.
Allowing Mother Nature to water in your granular fertilizer is a clever way to conserve water and time. However, I don’t recommend this strategy if there’s any chance of the rain being heavier than forecast.
Liquid Fertilizer Run-off
If it rains following an application of liquid fertilizer, the fertilizer will be diluted and flushed deeper into the soil. This isn’t a huge problem as long as the rain is light. However, heavy rain can flush liquid fertilizers so deep that plant roots can’t utilize them.
Rain that is so heavy that it pools or flows across the soil’s surface is bad news following the use of liquid fertilizer. Not only will the fertilizer be removed from the target area but it could cause harm elsewhere.
Risk of Run-Off to Waterways
There’s a good chance you’ve heard of fertilizer run-off, at least in passing. This problem has been gaining attention recently and can be exacerbated by heavy rainfall following fertilizer applications.
In short, fertilizer can have the same effect on aquatic plants — including algae — as in-ground plants. When these plants flourish, it throws off the balance of natural waterways. Fish and other aquatic life suffer as a result.
According to Scientific American, nitrogen and phosphorus are the largest contributors to this phenomenon. Select regions ban the use of phosphate-containing fertilizers altogether for this reason.
Contrary to popular belief, fertilizer run-off doesn’t just occur near rivers, streams, ponds, and lakes. It’s also possible for fertilizer to leach into groundwater reservoirs. This often occurs when the fertilizer is flushed down by heavy irrigation or rainfall before plants have time to consume it.
What Happens if it Rains After Fertilizing
When it comes to lawn care, rain is typically a good thing. But applying fertilizer immediately before rain can lead to some unwanted consequences — consequences which are not limited to the negative impact fertilizer run-off has on natural water systems.
Whether liquid or granular, washed-away fertilizers don’t need to leave your property to cause problems. For example, nutritional problems can arise when extremely potent fertilizers are washed into a garden bed containing less vigorous plants.
Also keep in mind that many fertilizers contain other products like insecticide, herbicide, or even grass seed. These ingredients may also be diluted or washed away by rain. Most lawn care chemicals dictate the amount of time that should pass between application and irrigation or rain to prevent issues.
How Much Rain Is too Much Rain
It can be hard to tell what qualifies as heavy rain when it comes to fertilizer. While I wouldn’t define too much rain post-fertilizing by a certain number of inches, there are a few key signs worth looking out for.
If there is enough rainfall to fully saturate the soil to the point of puddling or active run-off, there is a very high chance that your fertilizer has been disturbed. Granules may be carried off by running water. Meanwhile, liquid fertilizers can be washed away or diluted so much within the soil that they are largely ineffective.
Larger granules applied to the soil’s surface should still be visible the following rainfall. As long as you still see the expected number of fertilizer granules, the rain has had little effect on your previous application.
Unfortunately, it’s nearly impossible to tell if liquid fertilizer has been washed away. Even in agricultural settings, Iowa State University advises against “replacing” nitrogen that may have been washed away without careful consideration. To prevent overfeeding, I recommend waiting to re-apply until the original dose would be otherwise depleted.
Applying Fertilizer to Wet Grass
Unless the label specifically states otherwise, it’s typically safe to apply fertilizer to lightly damp soil. Fertilizing pre-watered soil can even decrease the risk of root burn in some cases. However, you should not fertilize over heavily saturated soil.
While granular fertilizers should not be applied before the rain, you also want to avoid applying them immediately AFTER it rains. I recommend waiting until the grass blades have completely dried (this rule also applies to other water sources like morning dew or sprinklers!).
If you apply granular fertilizer to wet grass, many of the granules will stick to the blades. This prevents the fertilizer from reaching the actual soil and increases the chance of foliar fertilizer burn. Lightly watering the lawn after fertilizing will flush the granules down to the soil where they belong.
Note that there are some granular fertilizers specifically designed for use on wet grass. These products usually contain herbicides that work by sticking to dampened weed leaves. Always follow the manufacturer’s instructions when applying such fertilizers.
In my opinion, it’s fine to use liquid fertilizers on wet grass as long as the soil is not completely saturated. The grass will be moistened when you apply the fertilizer anyway.
Applying liquid fertilizer to already saturated soil is not recommended. Since the soil is already at full capacity, there’s no way for the fertilizer to penetrate below the surface. This means the grass will have a difficult time accessing the nutrients and the chance of run-off will be greatly increased.
Best Time to Fertilize Lawn: Before or After Rain?
The best time to fertilize your lawn is after rain. Ideally, you should wait for a day or two after heavy rain before applying fertilizer to the area. This will allow the grass blades to dry and give the soil enough time to drain.
While it’s true that light rain can be beneficial after applying granular fertilizer, it’s hard to predict exactly what Mother Nature has in store. Choosing to only apply feed grass after rain will minimize the risk of run-off and ensure no fertilizer goes to waste.
Fertilizing Vegetables Before or After Rain?
Turf grass is not the only plant you should avoid fertilizing before heavy rain. I also recommend waiting to apply fertilizer to vegetables and flowering plants if rain is in the forecast.
The environmental consequences of fertilizing before heavy rain are more serious for in-ground beds than containers. Fertilizer applied to in-ground vegetable beds is more likely to run off and enter nearby water systems when it rains. However, heavy rain can and will wash away fertilizers applied to outdoor containers as well.