Conifer Needles Against Slugs

Question

My hostas are attacked by slugs and I have tried many things to control them with variable amounts of success. This year, I’m trying pine needles as a mulch around my hostas. I’ve heard they can be quite efficient. However, will using pine needles change the pH of my soil and, if so, will it harm my hostas?

Jacques Cayer

Answer

A mulch of spruce or pine needles can help keep slugs away to some degree. Apparently, they dislike crossing sharp surfaces and, according to that belief, pine needles would an irritant for them. At least, that’s the explanation I’ve been hearing to explain why slug numbers seem to drop when conifer needles are used as mulch.

While that is possible, I still can’t help wondering if the explanation isn’t elsewhere. Might not the explanation be that if slug numbers drop, it’s because slugs are detritivores (animals which feed on dead organic material, especially plant detritus) and therefore find very little to eat in a mulch of conifer needles?

But I suppose the reason why is of lesser importance. People do report a serious drop in slug numbers when they use a conifer mulch, like pine straw. And if it works for you, go for it!

Conifer Needles Don’t Acidify Soil, Or at Least, Not Much

On the other hand, the popular belief that conifer needles acidify the soil is just not justified.

The acidifying capacity of the needles has been greatly exaggerated. It’s true that conifer needles are acidic when they are fresh from the tree and still green, but as they decompose, and turn brown, their pH (level of acidity) changes and they become increasingly alkaline. Typically, by the end of the process, their pH is around 6.8 … about ideal for gardening in general and certainly perfect for hostas!

But even this information, while encouraging, does little to change your situation. The thing is, the pH of a mulch (that layer of organic matter that the gardener places on top of the soil) has very little affect on the pH of the soil below it. So little is to not be worth mentioning. Even after 20 years of applying a mulch, whether acidic or alkaline, usually the original pH will not have changed at all. It’s like two independent worlds.

There is no reason why you couldn’t have an acidic mulch and an alkaline soil. Or vice versa.

Also, before blaming conifer needles for acidifying the soil, you first have to know what the pH of the original soil was. Most conifers (pines, spruces, firs, etc.) prefer acidic soils and therefore settle in areas where the soil is naturally acidic.

It’s not conifers that make the soil they grow in acidic. By far the most important factor influencing soil pH is the pH of the bedrock in the area. Soils tend to be acid where bedrock is acid and alkaline where it is alkaline. Of course, other factors, such as acid rain, certainly have some influence, but if your soil is acid, you can usually blame the bedrock… and almost certainly not the mulch!

Hostas Like It Acidic

Note that hostas prefer a pH of around 6.5 to 7.5, which is slightly acidic to slightly alkaline, but will tolerate an acidity as low as 5.5. If your soil is already at a lower pH, it might be worth making it more alkaline with an application of lime.

Benefits of Conifer Needle Mulch

Using pine needle mulch has a long list of benefits, including the following:

• Often readily available and free;
• Helps keep soil moister;
• Prevents the germination of weeds;
• Adds nutrients to the soil;
• Moderates soil temperature;
• Keeps plants and fruits clean;
• Stays light and airy without forming an impenetrable crust like some mulches:
• Is fairly stable and does not tend to wash away in heavy rains;
• Holds securely on slopes;
• Lasts a long time;
• Creates a beautiful effect in the flower bed, one that highlights the plants.

Good Hostas Don’t Have a Problem With Slugs

And by the way, why do you have to protect your hostas from slugs? Some hostas, especially old varieties, were very prone to slugs, but most modern varieties no longer have any problems. Consider replacing your holey hostas with cultivars like ‘Sum and Substance’, ‘Invincible’, ‘Canadian Shield’, etc. Here’s a list of 400 more slug-resistant hostas!