No grass if it has been treated with any herbicides at all. That one is vital.

Also it has to be well dried first before applying it and needs to be kept back from the stems of the plants and laid on in thin layers. It tends to mat down and it is very high in nitrogen and gets quite hot as it decomposes so it can burn young plants.

If you read thorough some of the other many mulch discussions here you'll find more info using grass. And don't forget there is a Soil & Mulch forum here too.

Pine needles aka pine straw is good stuff as long as your soil isn't already overly acidic as they are high acid and can acidify soil as they decompose. They are quite slow to decompose - as in years not months.

These are just some of the reasons why straw is considered the ideal garden mulch - doesn't mat, decomposes as the garden ages, improves soil texture, no heating, no burning of plants, pH neutral, etc. But it isn't available or affordable to some so the many other options, while perhaps second choice, are less expensive and certainly better than no mulch at all.

Dave

I've used very thin layers of completely DRIED grass for years with out it heating up. They're perfect for square foot gardening boxes or even lightly in containers. If you use your own clippings you'll know they're herbicide free.

Dry them out on a tarp on nice days & cover up at night so no dew forms. When they're light brown and smell like hay you can use them without getting slimy in the garden.

I often dry the clippings from our first mowings of the year because the lawn is very green & healthy then and store until the soil is warmer in later spring. If I wait until summer to mow the lawn isn't as lush and green, so the clippings aren't as nitrogen rich. Plus if I mulch too early on the soil stays too cool, so I wait.

Sometimes, our grass has gone to seed, but isn't ripe, so no seedlings sprout. If you had brown grass heads you might get reseeding.

I take out a few handfuls & put in a bucket to carry to the garden to spread about a tablespoon at a time with 2 fingers around, but not touching the stems of young plants. Be sure there are no lumps or clumps, just a LIGHT layer that overlaps.

I add more about every few weeks as needed to keep the soil covered and moist. As the plants grow you'll know when you need more. I use my finger or a chopstick stuck in the soil to check the moisture below. By mid-summer when we've had no rain for a month I've built up thick layers that no weeds can penetrate and the soil stays nice underneath.

Larger plants get more, but make sure THIN layers of dry grass at a time. Probably still no more than a half inch at a time. Don't ever add green clippings on top unless you're very careful to sprinkle little bits like teaspoons of them VERY THINLY during warm dry weather to dry in place. The dry grass doesn't attract slugs more than other mulch, but I do use iron phosphate baits & patrol for slugs with a stick or shovel and soapy bucket handy.

For tomatoes and large zucchini plants I prefer to use an older bale of straw that clumps together and I can pull off sheets of straw to place side by side. Grass clippings fill in the gaps. New straw is a bit slippery for walking upon, but also works if that's what you have.

I haven't yet mulched our boxes because we've had 2 weeks of dry weather until yesterday. Today it's raining non-stop soaking everything nicely, so I only went out for slug duty this morning and dodged puddles in the driveway. The forecast is for rain until Friday, so when there is a break I'm going to jump on it to get the mulch applied. I have 2 trash cans full still from last year, so have plenty.

Also be sure to water gently so you don't move your mulch layer. Mulch goes over the soaker hoses. Spread after watering or a rain to keep moisture in. Once dried out it's hard to rewet the soil when you have that mulch layer.

We have dry summers, so mulch really reduces the watering needs and keeps our lettuces tasty.

I love using white pine needles, they're soft and fluffy and work great. I would avoid using needles form Scotch or Austrian simply because they are sharp and pointed when dry, and aren't too pleasant on the hands in a vegetable garden where you'll be coming into regular contact with them. They're better, IMHO, around trees and shrubs where you don't need to work frequently.

I imagine if you're buying "pine straw" at a nursery, it's some long needled, soft type, not something hard and prickly. Or, at least I would hope so.

I use pine straw exclusively for mulching and lining garden paths. I refresh some of the pathways a couple times a year, because it does break down if you're walking on it on a regular basis.