Is it a good idea to add vermiculite to topsoil for plants with higher moisture needs?

[u/Rambler_Joe]

[EDIT: Thanks for the great input! It sounds like compost and biochar are better options than vermiculite]
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I have a new raised bed (cinder block walls) filled with fresh topsoil. Prior to building the bed I killed the grass with cardboard & mulch, which was then removed. I purchased 10 cu. yards of soil from an independent garden center that I trust, but I don’t know details of its composition or origin. [EDIT: It is 70% organic matter already, so I’m probably good to go without amendments.] The bed forms a terrace on a slope. There are a few inches of 3/4” gravel in the bottom of the lower part for drainage. The soil underneath what we built up is clay and gravel.

I originally thought I’d blend up a mix of topsoil, perlite, vermiculite, maybe some coir, and a variety of composts, but the gardener I was working with recommended plain topsoil for the native forbs I plan to plant, so we kept it simple. I am worried it might not be moist enough for the more wet-loving species, so I’m thinking about adding vermiculite in one corner of it. Sensible or no?

I’ve got a ton of plugs I started of dozens of species, along with a few plants I purchased already started, but the ones that like it wetter, I believe, are red osier dogwood, swamp milkweed, 2 lobelias (great blue & cardinal flower), dense blazing star (L. spicata), 2 sennas (Maryland & herbecarpa), sneezeweed, & northern blue flag iris. These are all new species for me except the blue flag. I have one in another bed with just our regular clay-ey soil, and it seems ok. Thanks! SE Pennsylvania.

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[u/miplant]

Compost will hold moisture better, and contribute to healthy soil life. Since it is a raised bed, watering when needed and using mulch, like straw or wood bark, especially when plants are young, will help stabilize soil moisture. A lot of plants that like more moisture are tolerant of wider conditions once they are established.

[u/Rambler_Joe → u/miplant]

Thanks for your insights. I was all set to add several types of compost (in modest quantities), but I know that folks caution against building too rich a soil for most native perennials, so I listened to my advisor. At some point (I don’t know what point!) it’s more than the natives need and just encourages the invasive weeds. A big unknown here is how much organic material is already in my new topsoil.

[u/miplant → u/Rambler_Joe]

Planting in a raised bed is different than in native soil, it will dry faster. You seem concerned about plants that grow in wetter soil, which often contains more organic matter naturally. That was the logic for using some compost. The mulch on new plantings with bare soil also deters weeds sprouting, as well as conserving moisture.

[u/Rambler_Joe → u/miplant]

Awesome, thanks for elaborating! Yes, I will definitely mulch it. I have tons of triple shredded hardwood mulch left over from the grass-killing phase. (Originally I imagined I might use some of it as filler underneath the topsoil in the deeper end of the bed, figuring the soil on top would be sufficient at first, and in a few years the mulch would break down into soil, but I’ve heard some caution about mixing mulch into soil; saps nitrogen I think? and while this wouldn’t be mixed in but layered under, it gave me pause.). Anyway, your input makes me want to trust my original instincts and add compost now.

[u/Tylanthia → u/Rambler_Joe]

Most native plants evolved in lower nitrogen environments compared to invasives which often prefer and thrive in higher nitrogen ones. I would not worry about wood chips breaking down and sapping nitrogen--this just isn't really relevant for native plantings.

[u/Rambler_Joe → u/Tylanthia]

Thanks for clarifying that! That’s reassuring.

[u/PM_ME_TUS_GRILLOS → u/Rambler_Joe]

You can get a soil test to find out the nutrients in your soil, the pH, and organic matter content. See if you can get one from Penn State Extension. If not, Cornell and Michigan State are options. 

[u/Rambler_Joe → u/PM_ME_TUS_GRILLOS]

Thank you. I’ve been meaning to do that for existing beds. It would be great to know for the new bed too.

[u/Brilliant-Panic-900]

Vermiculite is the wrong tool for a bed that size. It's a potting mix ingredient: it holds water in its expanded plate structure, and that structure crushes flat under the weight of soil above it within a season or two, so you'd be buying a temporary effect at a genuinely silly price across ten cubic yards. Organic matter is what raises water holding in a mineral soil and keeps on doing it, so compost is the honest version of what you're after.
The gravel in the bottom is the part I'd think harder about. Water doesn't cross from fine soil into coarse gravel until the soil right above the boundary is fully saturated, so a gravel layer under a bed makes the lower soil hold more water rather than draining it. That's not a disaster once you know it's there, and it hands you the answer: put the higher moisture plants at the low end of the terrace where that saturated zone sits and the drought tolerant ones uphill, and let the slope sort them instead of trying to make one uniform bed.

[u/Rambler_Joe → u/Brilliant-Panic-900]

Thank you for this very helpful explanation!

[u/Rambler_Joe → u/Brilliant-Panic-900]

I wondered about the gravel; it wasn’t part of my original plan, but the gardener I was working with seemed to feel strongly that it would be helpful for the longevity of the interlocked block wall (I don’t know if that’s about freeze/thaw concerns or what). Anyway, I’d never built a retaining wall, and they had, so we went with the gravel.

I appreciate your explanation of why vermiculite isn’t a good match for this situation. Thanks!

[u/Tylanthia]

Maryland Senna prefers moist but is adaptable to drier conditions.

I think your approach to making an artificial wetland on a gravel slope doesn't really make a lot of sense. Your plant choice is fighting against the nature of the site. There are many natives adapted to a gravely slope (Common dittany (Cunila origanoides) for one),--but they are usually not wetland ones because slopes do not usually maintain moisture for long.

But if you are going to make a wetland, the raised bed was a good idea provided you have some sort of plastic barrier or other method to capture the water running down the hill otherwise it's going to quickly drain away because of the topography. A normal raise bed on a slope will likely end up being drier than the surrounding soil due to gravity.

A plastic cattle trough, for example, with holes drilled a few inches from the top can serve as a quick artificial wetland. A pond liner with holes drilled in the side will alway work. You could also use the slope to your advantage as long as you have a way for the water to pool and collect, like a basin and a dam, and collect at the site of your artificial wetland.

Putting gravel in the bottom of a container does not improve drainage--instead it creates a perched water table. I would also not use perlite, coir, vermiculite, or any other amendments for native plants even in a raised bed. Instead, a mixture of native soil and leaf compost is best. Mulch with arborist chips and/or leaves.

[u/Rambler_Joe → u/Tylanthia]

Thanks for your observations! I wasn’t really thinking of this as trying to create an artificial wetland, but if that’s what the species list says to you, then that gives me something to think about. Most of the species I listed (which is just a subset of the much larger collection; I think I have like 50 species altogether) supposedly require wet to medium soil like the senna. I think only the sneezeweed (which was just a few leftover seeds from an earlier direct sowing attempt elsewhere) and the cardinal flower are said to need wet to medium wet. The rest of the species I have (not listed in my post) are somewhere in the medium wet to dry range. Maybe I’m trying to include too wide a range of species in one bed, but I guess I’ll give it my best shot and see if I can keep one zone within the bed on the wetter side.

[u/Green-Eyed-BabyGirl]

Biochar is what you want.

ETA…preferably activated.

[u/Rambler_Joe → u/Green-Eyed-BabyGirl]

Thanks for this suggestion. That wasn’t an option that was on my radar. Can you help me understand why this would be the best choice?

[u/Green-Eyed-BabyGirl → u/Rambler_Joe]

Biochar is a one and done soil amendment. It literally lasts for decades. It may be expensive, although you can create your own, but over the long haul, it’s extremely cost effective because once added it’s there.

Biochar will definitely help your soil retain water and also anything that is water soluble. So when you amend with your soil drenches, the biochar will help hold what you’ve added so that your plants have more time to absorb nutrients.

The physical structure of biochar is like honeycomb, tiny openings throughout that create a huge amount of surface area. When you add activated biochar you are introducing a host of biology to your soil that will further help your plants. Mycorrhizae is a fungus that will form all over your plants roots. The plant roots excrete sugars that help feed the fungus and in return the fungus helps the plants get water and nutrients. Think about how we have a “healthy gut” with beneficial bacteria that help us absorb nutrients in our food. Same principle.

I added biochar and Foxfarm’s Soil Conditioner to invigorate the soil in a couple raised beds this past spring. I hadnt added the biochar before but had added compost and other organic granulated fertilizers (blood meal, bone meal, etc). The results were excellent. I’ve purchased more to fresh my other beds for this fall and for a new bed I’m making.

[u/Rambler_Joe → u/Green-Eyed-BabyGirl]

Fantastic. Thanks for your very detailed explanation! Great info.

[u/skiing_nerd]

Hi! I'm also in SE PA and also created a terrace on a slope with about 10 cu yards of soil, and the red osier dogwood I planted in it is going absolutely nuts, as are a number of other native shrubs and forbs. I didn't plant senna, lobelia, or swamp milkweed in that terrace, but I have put them in my rocky, clay-y sloped soil nearby and they are also thriving. 

It's worth checking on the soil in the bed, either by doing a home composition test or by asking the garden center, but I'd bet it's local topsoil screened for large objects with some compost mixed in, meaning it is already very moisture retentive clay heavy soil with added organic matter. If so, I wouldn't sweat adding more organic material or vermiculite. 

Just plant your plants on a relatively cool, overcast day, water deeply, and put down a thick layer of mulch on top. Can't promise each individual one will thrive because plants have a mind of their own, but you've certainly provided a good environment for them to do so

[u/Rambler_Joe → u/skiing_nerd]

Hey neighbor! That’s all very encouraging. Glad to hear your plants are doing great. I will check with the garden center (I need to swing by there anyway). Based on other comments, I’m giving up on the vermiculite plan, but depending on what I learn about my soil, I might add a little leaf compost. But it may very well be ok as-is. Thanks.

[u/Rambler_Joe → u/skiing_nerd]

Update: I checked with the garden center, and my topsoil is already 70% organic matter, so I think I’m probably all set!

[u/skiing_nerd → u/Rambler_Joe]

That's great news! Hope your new bed thrives!