For growing Penstemon in the northeast, dig in gravel or use as mulch?

14 posts / 0 new
Last post
sf2bos2prov
Title: Member
Joined: 2018-10-04
For growing Penstemon in the northeast, dig in gravel or use as mulch?

I'm growing several European hybrid strains of Penstemon  (Twizzle and Rondo) that I started from seed and now have growing nicely inside under growlights.  I also winter-sowed several species in milk jugs which surprisingly are doing reasonably well (though still only tiny seedlings) considering they're growing in regular moisture-retentive potting soil: P. pallidus, P. grandiflorus, P. richardsonii, P. serrulatus, P. tubaeflorus, P. cardwellii, P. cardinalis.

I also purchased several plants of European-bred Penstemon hybrids (Arabesque, Red Riding Hood, Mesa).  I also have on order plants of the straight species Penstemon ovatus and P. smallii. 

So I have a lot of Penstemon in progress, with a diverse mixture of types.  I figure I'll be ready to plant some of these in a week or two; others, not until next month.  

However, I just don't have the ability to create any sort of rock garden right now and need as best as I can to rely on my moist, organically-rich New England soil.  My question: if I did want to give these seedlings some of the fast drainage they want - especially the midwesterners - would it better to dig gravel (or sand?) deep into my soil, or should I spread a layer of gravel as mulch on the surface?  Thanks for any thoughts.

RickR
Title: Moderator
Joined: 2009-09-21

For drainage, you want dig in larger aggregate - gravel, if you want to try to transform your soil.  Penstemon can take somewhat richer soil IF it is very dry, not a likely scenario for you, I would think.  They are alway going to prefer a very lean soil.  P. hirsutus and cobaea seem to tolerate rich soils better, but for everything else, I would rather dump a sand and gravel mix right on top of your existing soil, and have them grow in that pure mineral mix.  Sand or gravel as as mulch over rich soil isn't going to help.

 

Grandiflorus and pallidus are native here in Minnesota, and they grow in pure sand or in rock outcrop crevices.  The hybrids should be able to tolerate richer heavier soils.

Rick Rodich    zone 4a.    Annual precipitation ~24 inches near Minneapolis, Minnesota, USA

sf2bos2prov
Title: Member
Joined: 2018-10-04

[quote=RickR]

For drainage, you want dig in larger aggregate - gravel, if you want to try to transform your soil.  Penstemon can take somewhat richer soil IF it is very dry, not a likely scenario for you, I would think.  They are alway going to prefer a very lean soil.  P. hirsutus and cobaea seem to tolerate rich soils better, but for everything else, I would rather dump a sand and gravel mix right on top of your existing soil, and have them grow in that pure mineral mix.  Sand or gravel as as mulch over rich soil isn't going to help.

 

Grandiflorus and pallidus are native here in Minnesota, and they grow in pure sand or in rock outcrop crevices.  The hybrids should be able to tolerate richer heavier soils.

[/quote]

 

Thanks, Rick. I want to make sure I understand the difference you're making between dumping a sand/gravel mix on top of my existing rich soil and planting the plant in that and having the plant grow directly in that, versus using the gravel/sand mix as a mulch over my rich soil. I generally think of the two as the same thing. Is the difference you're making in the amount of the sand /gravel mix that I'm putting on top of my soil? If I dump 5-6" and plant in that, that's essentially growing the plant in that mix, as the bulk of the growth process takes place in that mix; whereas if I dump, say, only 2-3" that's just using it as a light mulch and the bulk of the growth is still taking place in my rich soil underneath the 2-3" mulch layer which is not ideal for anything. it's that the difference you're drawing of? Dumping 5-6" on top of my existing soil would be a reasonable substrate for pallidus and grandiflorus?

And both of the two scenarios are separate from digging a sand/gravel mix directly into my rich soil which would be ok for hirsutus and cobaea. But not for pallidus and Grandiflorus which want a more pure gravel / sand?

Thanks for helping me understand this. 

Near Boston, MA, zone 6b.  Average annual rainfall = 44 inches

RickR
Title: Moderator
Joined: 2009-09-21

Yes, what you are saying is exactly what I mean.  But alpine and prairie plants have very deep root systems. I would go a minimum of eight inches of sand/gravel on top, instead of 5-6.  In the long run, there will be less maintenance (weed pulling) with deeper 12+ inch mix.

 

This is penstemon canescens (very close to hirsutus) seedlings that are less than one year old, AND growing in a pot.  Roots would be even longer if they were in the ground.

And if you dig into your existing soils, I wouldn't even bother with sand.  You already have enough small particle materials.  Use only gravel, and if you want to incorporate 1-2 inch size aggregate, that would be even better as an addition with the gravel.

Rick Rodich    zone 4a.    Annual precipitation ~24 inches near Minneapolis, Minnesota, USA

sf2bos2prov
Title: Member
Joined: 2018-10-04

Those P. canescens seedlings haven't bloomed yet, have they?

For starters, I do have a couple of P. cobaea seedlings in 7" pots of that gravelly mix you recommended back in February.  Maybe I'll let them grow in the pot until September. Then I'll find a small spot in my garden to experiment where I'll dig in both pea gravel and turkey grit (I think that's about 1" aggregate granite) into the soil and plant them there.

Near Boston, MA, zone 6b.  Average annual rainfall = 44 inches

RickR
Title: Moderator
Joined: 2009-09-21

The pic shows P. canescens seedlings unpotted in the late summer.  Certainly not at a blooming stage.

 

#4 is Turkey grit.  It really doesn't provided any more air space in the soil than a grevel of the same size, but because of its angular surface, it does provide somewhat better excess water drainage when mixed with smaller aggregates, like sand or humus,. 

Rick Rodich    zone 4a.    Annual precipitation ~24 inches near Minneapolis, Minnesota, USA

sf2bos2prov
Title: Member
Joined: 2018-10-04

OK, I have plenty of turkey grit so I'll experiment with that. 

One thing I'm noticing in the pots I started in February is that I might have used more sand than I should have in making the potting mix. Your suggestion back on February 13 was for 50% gravel, 20% regular potting mix and the rest chicken grit, turface and ouster shell. That's pretty much what I did and you didn't mention sand but somehow I got it into my head that I needed to add sand. So I added some percentage of sand in the mix, and with all the rain we've been having in Boston this spring, that does seem to be keeping the mixture more moist than I would like. Maybe it's also because I used 7" pots and I know you mentioned a few months ago that larger pots do hold in moisture longer than smaller ones.

So the seedlings have been growing quite slowly - most are still not out of the cotyledon stage - and I wonder if it's because of excess moisture captured and held in by the sand. We've been getting a lot of rain and not much sun, and with the percentage of sand the whole mix stays fairly moist - more, I think, than is good for him. 

I think next year I'll use less sand and smaller pots.

interestingly, and separate from all these other science experiments, in a separate area of my yard I have an old abandoned sand box left by the former home owner, still with sand in it. I wonder if I might be able to re-purpose that into a kind of informal rock garden for Penstemon. Hmm.

Near Boston, MA, zone 6b.  Average annual rainfall = 44 inches

RickR
Title: Moderator
Joined: 2009-09-21

When you say you have plants not out of the cotyledon stage, do you mean no mature true leaves have formed yet?  They need some fertilizer.  Use a quarter strength liquid fertilizer like regular Miracle-gro, to start.  If they respond favorably, even the tiniest bit, then the next time give half strength.

  If tiny seedlings are getting too much water, they don't have the stored carbohydrates to endure very long, like older plants do.  I certainly would have expected them to expire by now if they were overwatered.  Remember too, you can always bring the pots under house eaves when it rains.

If the 7 inch pots are also 6 plus inches deep, you ought to have the drainage at the top of the pot that you need, at least for growing in the top 2 inches for sure.  Excess water that would cause problems will be pulled down by gravity and capillary action of the media below.  But when you get down to the bottom inches, thats where you will have the perennial wetness.

Rick Rodich    zone 4a.    Annual precipitation ~24 inches near Minneapolis, Minnesota, USA

sf2bos2prov
Title: Member
Joined: 2018-10-04

[quote=RickR]

When you say you have plants not out of the cotyledon stage, do you mean no mature true leaves have formed yet?  They need some fertilizer.  Use a quarter strength liquid fertilizer like regular Miracle-gro, to start.  If they respond favorably, even the tiniest bit, then the next time give half strength.

  If tiny seedlings are getting too much water, they don't have the stored carbohydrates to endure very long, like older plants do.  I certainly would have expected them to expire by now if they were overwatered.  Remember too, you can always bring the pots under house eaves when it rains.

If the 7 inch pots are also 6 plus inches deep, you ought to have the drainage at the top of the pot that you need, at least for growing in the top 2 inches for sure.  Excess water that would cause problems will be pulled down by gravity and capillary action of the media below.  But when you get down to the bottom inches, thats where you will have the perennial wetness.

[/quote]

Yep, there are almost no true leaves yet with any of my seedlings, about a month after germination for most species.  The seeds were planted in the pots outside in late February.  The interesting thing is that most of the same seeds that I winter-sowed in regular potting mix in plastic jugs are showing true leaves.  So maybe your thought about their needing fertilizer is correct. But how is the situation I have them in there any different from what they will encounter in the garden, or growing in the wild? I don't think they'll be getting much nutrients there, will they?  In other words, why do they need fertilizer?  I think of Penstemon as not needing or requiring, or even wanting, much fertilizer. 

I did realize a few weeks ago that unlike the plastic jugs with potting soil I didn't have these pots of gravel in a full-sun part of the yard; maybe that's another reason for their slow growth.

Another possibility: I looked at the medium just below the surface and see that it's more dense and fine than the course gravel at the surface.  Maybe I used too much sand and/or seed starting mix and therefore the drainage and porosity are not as good as it could be.  Sand sure does hold water.

I just measured and I see that the pots are 6" in diameter, not 7".  They're 7" tall.

Here are some photos. 

Penstemon virens
Penstemon ovatus
Penstemon grandiflorus
Penstemon richardsonii
P. richardsonii in potting soil
Maybe the medium in the pots is too dense

Near Boston, MA, zone 6b.  Average annual rainfall = 44 inches

RickR
Title: Moderator
Joined: 2009-09-21

The growth difference between the pots and the jugs is also partly due to temperature, but I think it is mostly nutrition.  The jugs can heat up in the sun, even if there tops are off, and more so if the tops stay on.

 

Yes, I really think fertilizer will help.  When I say repond in the tiniest bit, I mean that they may not actually grow, but you should see more robust color, and the cotyledons have more substance.

 

In the wild, plants in general do grow slower and have tougher times than when we try to grow them.  A plant of the same size in the wild is usually much older than one grown in captivity.  So how do they survive?  Remember that the percentage of survival in the wild is far far less that what we eperience in a pot or garden, but wild plants start from a bank of seeds incredibly large in comparison.  There is a whole lot more "survival of the fittest" in the wild than in any garden or pot.  And yes, maybe they do survive on less nutrients, but the live longer, and benefit from other good things in the environment that keep them going better.  Things like drier overall conditions and more air movement that allows for less pathogen infections to fight off, greater day-night temperature fluctuations, natural rain as opposed to well or city water, perhaps some beneficial mycorrhizae symbiosis, or benefits from plant diversity in the environment.  Always be looking at the big picture, and how different environmental characteristics affect the plant, and affect how the plant responds to other conditions.

Rick Rodich    zone 4a.    Annual precipitation ~24 inches near Minneapolis, Minnesota, USA

sf2bos2prov
Title: Member
Joined: 2018-10-04

ok, I just fertilized the seedlings in my pots of gravel, using Foxfarm Big Bloom.  I used half the amount recommended for seedlings, which itself is half the amount recommended for plants.  I'd held off for a few days because it's been raining a lot here in Boston and I didn't want to add more water to what's probably already very wet gravel in those pots, but we now have a few sunny days ahead of us and I figured it'd be a good time.  I didn't use a lot of water since I can't imagine the roots are very deep right now.

Part of me is concerned that such slow growth at this early stage could stunt their future growth (I was expecting more than cotyledons after a month after germination) but it sounds like you're suggesting that wouldn't necessarily be the case.

I can imagine that some of the midwestern and western species I have in those pots are not particularly happy to be starting their life in a very rainy New England spring, but other than this slow growth they don't seem to be showing signs of diminishing enthusiasm.

Near Boston, MA, zone 6b.  Average annual rainfall = 44 inches

Pages

Log in or register to post comments