There’s a gardening myth that using pine needles as mulch will acidify the soil. This may be detrimental to some plants, so it makes sense to be cautious about using it. However, is it true?
Pine needles are safe to use on any plants as mulch. It does not acidify the soil permanently because the microorganisms in the ground will decompose it, effectively neutralizing its low pH. It also doesn’t benefit plants that thrive in acidic soil, such as blueberries and rhododendrons.
Below, I discuss this phenomenon further with helpful tips for using pine needles mulch:
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Table of contents
Do Pine Needles Make the Soil Too Acidic for My Plants?
The common gardening belief about pine needles or straws is that they make the soil too acidic for plants. This is simply false. But why?
Green pine needles on the tree have a pH of 3. When these fall to the ground, they have been found to change the ground’s pH slightly in tiny amounts. This is insufficient to make a significant difference that may harm plants’ growth.
Furthermore, as the pine needles/straws turn brown and break down, the microbes in the soil neutralize their pH, shifting the balance back to normal. So whatever slight acidity there is in the ground from the pine needles/straws is gone! This means it is also short-lived and not as beneficial as people think it would be for acidic-loving plants like rhododendrons and blueberries.
But why are plants growing under or near pine trees not thriving? Growing under a tree is already big competition since they have many large roots that suck up more nutrients, leaving very few for other plants. So the acidity of the fresh pine needles was never the cause of the nearby plants’ stunted growth.
Does this mean using pine needles on plants is useless? Of course not! They still work great as mulch for decorative purposes and/or plant protection. Just don’t expect it to have magic powers to make your ground more acidic.
Helpful Tips on Using Pine Needles as Mulch
For those unfamiliar with the term ‘mulch’, it is a material you place over the soil on the ground to protect and/or improve it for a plant’s development. Nature does this all the time with fallen leaves! With pine needles, you’ll have to keep in mind a couple of things like the following:
- Pine needles need topping up every 1-5 years. You don’t have to make frequent additions because this mulch is known to decompose slowly in colder and dryer regions. So they are best used for perennial plantings, mainly to protect plants over the winter by retaining moisture and heat. However, if you live somewhere wet and warm or they are shredded into smaller pieces, the pine needles will deteriorate faster.
- Use shredded pine needles on annual plantings. Since annuals are short-lived plants that grow and die within a year, using mulch that decomposes fast is advisable. That way, you can rake it into the soil and improve it as it breaks down.
- Keep the pine needles mulch layer to 2-3 inches. Any more than that, you may end up restricting water and airflow into the soil, causing poor plant growth and increased risk of pest & disease problems.
- Water the ground deeply before covering it with pine needles mulch. This is particularly important for the ones that break down slowly. You want to ensure the soil is well-saturated before it has mulch to keep the moisture in.
- Rake the pine needles mulch to break up any mats every year. This is especially helpful if the mulch is unshredded. Even if the material has yet to start degrading, freshening it up will ensure water and air can still pass through.
- Discard leftover pine needles mulch into your compost pile. This is the best way to recycle your used pine needles if you want to add new mulch around your plants. But ensure to shred it, if not already, and only use 10% of it in your bin or heap.
Frequently Asked Questions about Pine Needles
Pine needles can be used as mulch around tomato plants, provided it is shredded. These will break down faster, which is more helpful when applied around annual plants like tomatoes. However, you can skip the shredding step if you’re growing perennials.
As a whole, pine needles are useful as mulch. The material helps suppress weeds, retain moisture and heat in the ground, and prevent compact soil! However, it’s worth noting that they decompose more slowly than other mulches, so they are best used on perennial plants.
It is unnecessary to use mulch-like pine needles on houseplants because it doesn’t benefit them as it would for in-ground plantings. However, shredded pine needles can be used as a decorative cover for indoor plants. Just ensure not to place a thick layer so water and air can flow in easily.
It is beneficial to rake pine needles at least once a year to break up matting in the mulch layer, especially if it is littered with fallen leaves. Not doing this may restrict air and water movement into the soil due to the impenetrable mulch layer.
Pine needles mulch is not necessarily preferred by plants like hydrangeas, cucumbers, and roses. However, the material may still be valuable as mulch to protect the plants from weed issues. There’s nothing special about pine needles themselves – they won’t make the soil more acidic, but that doesn’t mean they are entirely useless.
Pine needles mulch is not harmful to use in a vegetable garden. As long as they are shredded, applied at 2-4 inches layer, and raked yearly, they are safe to use. The fear of using this material due to its supposed soil-acidifying powers and, thereby, killing off plants is just another gardening myth.
All plants can benefit from pine needles mulch, provided it is applied correctly. This includes keeping it at a 2-4 inches thin layer and raking it yearly to prevent compaction. Pine needles won’t make the ground significantly acidic because the effect is negligible and short-lived.
Compost excess pine needles if they are not needed as mulch. You should only use 10% of the material to the actual compost heap and ensure it is shredded thoroughly. This will encourage the pine needles to break down faster.
Pine needle mulch is troublesome to use on annual flower beds. This is because the material has a slow decomposition rate, which is useless to annual plants like flowers. However you can always shred it, but it may still be too much work. Only use it on perennial flowers and plants instead.
Final Words
All in all, no plants dislike having pine needles as mulch. It is harmless to most and not extra beneficial to acidic-loving plants. But despite that, they are still one of the cheapest and longest-lasting mulch to use on perennial plants. So next time you see pine needles/straws being sold, grab some and apply them around your plants. Happy planting!
References:
- Pine needle mulch #735128 – Ask Extension. (n.d.). Ask2.Extension.org. Retrieved September 21, 2022, from https://ask2.extension.org/kb/faq.php?id=735128
- Will, M. J. (2020, July 13). Will Pine Needles Make My Garden Soil pH Too Acidic? (No, It’s a Myth). Empress of Dirt. https://empressofdirt.net/pine-needles-soil/
- Brasuel, S. (2017). Pine Needles As Mulch. https://mgeldorado.ucanr.edu/files/270887.pdf
- Jun 20, F. | H. 1604 | U., & Print, 2016 |. (n.d.). Mulch. Home & Garden Information Center | Clemson University, South Carolina. https://hgic.clemson.edu/factsheet/mulch/
- Using Mulch in the Garden. (n.d.). Hortnews.extension.iastate.edu. Retrieved September 21, 2022, from https://hortnews.extension.iastate.edu/using-mulch-garden
- GUIDE TO MULCHES. (n.d.). Retrieved September 21, 2022, from https://s3.wp.wsu.edu/uploads/sites/2076/2017/07/C075-Guide-to-Mulches-15.pdf
- County, C. (n.d.). Cooperative Extension Types and Uses of Mulch in the Landscape. Retrieved September 21, 2022, from https://chemung.cce.cornell.edu/resources/mulch-in-the-landscape
