This is my experience with how to plant warm-season vegetables dangerously close to the last frost date and not kill them. This year, I've used walls of water and black plastic mulch to protect seedlings.
Prologue: My seedlings were all at least 5 in high (some larger as I was too optimistic about the last frost) when I transplanted them into the garden. I started some plants from seed indoors and bought some seedlings from a local nursery, and others online because I thought I wouldn't be able to find them locally.
Both walls and mulch help early in the season by raising the temperature around plants (walls of water) and raising soil temps (black mulch). Cold temps can stress plants, kill them (less than 40f can kill peppers, for instance), or reduce nutrient uptake resulting in slowed or stunted growth. We sure need it here at 5000ft, as it got into the 30's (F) at night well into mid-May.
Walls of water: They are the turquoise teepees in the pic, available at your garden store $7 per 3 walls. Ouch! But worth it if you insist on trying to improve upon nature as I do. One 'wall' is a plastic cylinder of connected cells that you fill with water. The water absorbs the sun's heat all day, and radiates it back to the plant at night. The package says you can transplant plants up to 6-8 weeks earlier using walls, but I'm a bit skeptical of that, especially when 6-8 weeks earlier means frost/snow and many vegetables can't handle temps below 40 or 45F (that's a claim of ~10F temp increase). However, I think you can get closer to these figures by completely encasing the plant: if you only fill the cells 2/3-3/4 full of water, the cylinder will lean in on itself to form the teepee shape you can see in the pic. When closed off like that, it forms a mini-greenhouse around the plant. I'm using the teepees with black mulch, and when I open the top and stick my hand in, it's a regular sauna in there. Steamy and hot. The steamy part is becoming a problem for my squash as it's providing a quite hospitable environment for powdery mildew, but that's another post.
Now that it's mid-June - warmer, and some of the squash plants are getting bigger - I want to open the teepees and reposition them so they are cylinders, open on the top for more air-flow and space within. However, I found that the teepees protected many plants from early flea and cucumber beetle infestations - which are not over - so I'm torn as to exactly how to proceed...
Black Plastic Mulch: Basically, an impermeable soil covering that absorbs the sun's heat (because its' black) and transfers the heat to the soil, raising soil temps to help warm-season vegetables get a head start. Also helps with moisture retention and weed control, as do all mulches :) (Other mulches, like straw, are better later in the season, once the weather has warmed up, since they help keep soil temps more stable i.e. cooler)
Trash bags, the poor man's mulch! Actually, I first bought some black plastic from the local hardware store. All they had was 4mil and it was quite thick. I read several places that the mulch needs to have good contact with the soil in order to transfer heat it's absorbing from the sun, and 4mil was too thick to wrap around all the bumps in the soil. Then I realized black trash bags were the perfect 1-2mil thickness I had been looking for. So, I now have 2 beds covered in 4mil and the rest in 1mil (total: $7, not bad!) We'll see if it makes a difference, but I doubt it: all the mulch has lost contact with the soil since so many weeds are growing under it and pushing up on it(sickly, white weeds that will never see sun, heehhee). I'm finding that it's still quite the sauna under the mulch anyway.
When you are laying down the mulch, you'll need to cut holes for plants. I had already planted a few seedlings, so this involved some estimation of exactly where the hole needed to be to accurately accommodate the stem. Thankfully trash bags tear easily, although strangely, in only one direction... Secure the plastic to the edges of the bed with garden stakes so it lays flat and tight over the bed.
I put the walls of water up May 18 I think, when I first planted a few plants (tomatoes and a couple of peppers). I put the rest up May 25 when I planted squash and eggplant, and the tomatoes lost their walls then b/c I didn't have enough. I put the mulch down, in parts, the last couple of weeks of May.
Not sure how long I'll leave all this stuff on, but hopefully it's helping. I have a 'control' eggplant at my house (no need to get into the messy variables of house vs. garden, container vs. ground, okay?) that has neither mulch nor walls, and several 'control' tomatoes at my house that did not have black mulch like the garden ones do. The control tomatoes also didn't have a flea beetle infestation, but hey, that's why experimental writeups have a discussion section, for all the disclaimers and possible explanations for your unexplainable results ;)
Prologue: My seedlings were all at least 5 in high (some larger as I was too optimistic about the last frost) when I transplanted them into the garden. I started some plants from seed indoors and bought some seedlings from a local nursery, and others online because I thought I wouldn't be able to find them locally.
Both walls and mulch help early in the season by raising the temperature around plants (walls of water) and raising soil temps (black mulch). Cold temps can stress plants, kill them (less than 40f can kill peppers, for instance), or reduce nutrient uptake resulting in slowed or stunted growth. We sure need it here at 5000ft, as it got into the 30's (F) at night well into mid-May.
Walls of water: They are the turquoise teepees in the pic, available at your garden store $7 per 3 walls. Ouch! But worth it if you insist on trying to improve upon nature as I do. One 'wall' is a plastic cylinder of connected cells that you fill with water. The water absorbs the sun's heat all day, and radiates it back to the plant at night. The package says you can transplant plants up to 6-8 weeks earlier using walls, but I'm a bit skeptical of that, especially when 6-8 weeks earlier means frost/snow and many vegetables can't handle temps below 40 or 45F (that's a claim of ~10F temp increase). However, I think you can get closer to these figures by completely encasing the plant: if you only fill the cells 2/3-3/4 full of water, the cylinder will lean in on itself to form the teepee shape you can see in the pic. When closed off like that, it forms a mini-greenhouse around the plant. I'm using the teepees with black mulch, and when I open the top and stick my hand in, it's a regular sauna in there. Steamy and hot. The steamy part is becoming a problem for my squash as it's providing a quite hospitable environment for powdery mildew, but that's another post.
Now that it's mid-June - warmer, and some of the squash plants are getting bigger - I want to open the teepees and reposition them so they are cylinders, open on the top for more air-flow and space within. However, I found that the teepees protected many plants from early flea and cucumber beetle infestations - which are not over - so I'm torn as to exactly how to proceed...
Black Plastic Mulch: Basically, an impermeable soil covering that absorbs the sun's heat (because its' black) and transfers the heat to the soil, raising soil temps to help warm-season vegetables get a head start. Also helps with moisture retention and weed control, as do all mulches :) (Other mulches, like straw, are better later in the season, once the weather has warmed up, since they help keep soil temps more stable i.e. cooler)
Trash bags, the poor man's mulch! Actually, I first bought some black plastic from the local hardware store. All they had was 4mil and it was quite thick. I read several places that the mulch needs to have good contact with the soil in order to transfer heat it's absorbing from the sun, and 4mil was too thick to wrap around all the bumps in the soil. Then I realized black trash bags were the perfect 1-2mil thickness I had been looking for. So, I now have 2 beds covered in 4mil and the rest in 1mil (total: $7, not bad!) We'll see if it makes a difference, but I doubt it: all the mulch has lost contact with the soil since so many weeds are growing under it and pushing up on it(sickly, white weeds that will never see sun, heehhee). I'm finding that it's still quite the sauna under the mulch anyway.
When you are laying down the mulch, you'll need to cut holes for plants. I had already planted a few seedlings, so this involved some estimation of exactly where the hole needed to be to accurately accommodate the stem. Thankfully trash bags tear easily, although strangely, in only one direction... Secure the plastic to the edges of the bed with garden stakes so it lays flat and tight over the bed.
I put the walls of water up May 18 I think, when I first planted a few plants (tomatoes and a couple of peppers). I put the rest up May 25 when I planted squash and eggplant, and the tomatoes lost their walls then b/c I didn't have enough. I put the mulch down, in parts, the last couple of weeks of May.
Not sure how long I'll leave all this stuff on, but hopefully it's helping. I have a 'control' eggplant at my house (no need to get into the messy variables of house vs. garden, container vs. ground, okay?) that has neither mulch nor walls, and several 'control' tomatoes at my house that did not have black mulch like the garden ones do. The control tomatoes also didn't have a flea beetle infestation, but hey, that's why experimental writeups have a discussion section, for all the disclaimers and possible explanations for your unexplainable results ;)
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