Do Pine Needles Acidify Soil?

A close up of pine needles piled as mulch
Marie Iannotti

For years we've heard that pine needles should only be used around acid-loving plants. Since the needles themselves lean toward being acidic, it would seem to make sense to think they'd lend that acidity to their surroundings, but it's not true. Pine needles do not lower the pH and acidify the soil.

There are many things that influence soil pH, including climate and rainfall. However mulching materials are not worked into the soil, so whatever effects they may have remain at the surface and do not alter the soil around plant roots.

Are Pine Needles a Good Choice for Mulching?

Yes. Pine needles are generally a good choice for mulching, especially if you have pine trees in your yard or pine straw is availability locally.

What is Pine Straw?

Pine straw is the name given to pine needles when they are used as mulch. Pine straw and pine needles are the same thing.

Commercially sold pine straw is either long needle or short needle.

  • Long needle pine straw is from the Southern Yellow pine tree. The needles are an impressive 12 - 18 in. Long and an attractive coppery color when dry, they make a gorgeous mulch and are extremely slow to decompose. You also need less long needle mulch to cover a large area. It costs more per bale, but since you are using less, it generally costs about the same or less to do a job, than short needle mulch.
  • Short needle pine straw sold commercially if from either the Loblolly pine or the slash pine. These trees grow much faster than Southern Yellow pines, which made them popular with the timber industry. So popular that they were squeezing out the Southern Yellow pines. However Southern Yellow is making a comeback, probably because of the market for pine straw, so long needle mulches are once again becoming more available.

That said, I rarely see pine straw for sale in my area, despite the concentration of pine forests. But if you can get your hands on some, whether commercial products or your neighbor’s surplus, pine straw can be an excellent and inexpensive mulch option you can use on your yard.

Besides the cost savings, there are several reasons pine needles are a good choice.

  1. They are lightweight. There’s enough heavy lifting in gardening. Not only are they easier on the back, but they also won’t compact your soil.
  2. Unlike a lot of traditional organic mulches, pine needles don’t bring a lot of weed seed with them. And they block sunlight from reaching the seeds that were already in the soil, preventing them from germinating.
  3. Pine needles are slow to decompose, which mean they don’t need replacing a month after you spread them, as many other organic mulches do. But when they do eventually decompose, they enrich the soil.
  4. As with other mulches, they moderate the soil temperature, keeping it cooler in the summer and insulating it, when used as a winter mulch to prevent repeated freezing and thawing.
  5. Once they settle, they won’t float away in heavy rain. My driveway is always sprinkled with the wood chips I use to mulch the nearby beds. Wood chips love to float. Pine needles form a loose mat and stay put.
  6. Pine straw is often recommended for slopes and hillsides, because it allows water to get through to the soil and the pine needles help keep it there, rather than washing to the bottom of the slope and taking the topsoil with it. Of course, this is not fail-safe. Extremely heavy rains and flooding will still be destructive.
  1. Pine needles are a renewable resource. No trees are felled to collect pine needles. Pine straw happens.        

Are There Any Down Sides to Using Pine Straw?

Of course. Nothing is perfect.

I’ve already mentioned that availability can be a problem. But if you do find pine straw, be aware that it can blow around in windy locations, at least until it has had time to settle.

If it does not fully cover the soil, you will still get weeds, and weeding in pine straw is not particularly pleasant. It may look light and fluffy, but it is still needles.

Also, as with all dry mulching materials, pine straw is flammable. If wildfires are a problem in your area, pine straw is not a good choice for you.

However, I still find it a good resource and think it’s worth giving a try.

How to Use Pine Needle Mulch

Use it just like any other mulch. Spread it around trees and shrubs and use it in your garden beds. Most sources recommend a 3 in. layer, but 3 in. will settle down to about 1 ½ in. I’d recommend at least 4 in. and the more, the better. Because it is slow to decompose, you will probably only need to top it off with a couple of inches or so each spring.

Shake and toss pine straw, the way you would regular straw, so that it forms a fluffy layer. It will eventually settle and stay in place, despite rain or wind. It does this on the forest floor, and it will do it in your garden.