I have two greenhouses... one is a 20x60 hoop house that's basically for loml's tomato and hot pepper addiction. The other is a steel T channel unit with glass panels that's about 8x12. The glass greenhouse was bent up a little and missing a bunch of glass along the bottom from the previous owner apparently ramming it with a lawnmower into it.. but a little persuasion and some new glass and it's working quite well.
So seed starting... A greenhouse works for *some* seed starting but it's not a panacea and not IMHO perfect for all.
IMHO good seed starting requires several things, seed starting mix, potting tools, light, heat, moisture, . I'll cover what we've learned about all of those.
The seed starting mix is critical, you want a seed starting mix NOT a potting mix potting mixes are to coarse. We make our own because we start 100's (or 1000's) of plants so buying gets prohibitive.. I'm going to lay out what we use not because I think it makes sense at a smaller scale but because I think it might be useful to discuss the components.
- 50-60% coconut coir, I like this a bit more than peat as it's more PH neutral it's basically a bulking agent and holds some water. We treat ours with cal-mag (foxfarm liquid - 2 capfuls per 10lb dry block) to neutralize any residual sodium salts in it. If you buy dry blocks the cal-mag is critical and you should hydrate it first then stir that in and let it set for a while (we've had .. problems getting ahead of ourselves on treating it).
- 30-40% perlite, this is mostly to loosen the mix up. It doesn't hold a lot of moisture (some but not a ton).
- 10-20% vermiculite, this is for moisture retention pretty much. It also helps hold some nutrients in suspension.
For a 10 gallon batch of the above we add:
- 1 pint diatomaceous earth, this is mostly for bioavailable silica which makes the plants a lot sturdier. I think this would be good to add to most commercial mixes as well.
- 1 pint micronized fish bone meal (micronized is best), this was a key element in getting significantly more robust starts, it's a very available source of phosphate without being at all hot so it doesn't burn the plants at all. Our starts have monster roots now which was something we struggled with before starting to add this. Also IMHO not a bad idea to add to commercial mixes, especially for slower/fussy starts like tomatoes and peppers really benefit from this.
- 2 pints nutri-rich 4-3-2 granules which is basically pelletized feather meal. If you're using a commercial product you can likely skip this because it's probably already adequately fertilized.
- 1-2 pints worm castings for micronutrients (ditto on skip for commercial products)
Avoid the temptation for hotter additives like blood meal or similar. They can be ok in moderation when potting up but it's easy to burn the baby seedlings.
As for tools ... this is mostly about trays and such. IMHO if you're getting into this investing in decent quality trays is super helpful. We use 10x20 trays and do get the "heavy duty" ones of these for sure.. the flimsy ones will bend and break if you have a tray full of plants way to easily (we buy
https://www.greenhousemegastore.com/collections/trays-flats/products/1020-trays-heavy-duty also trays/inserts from bootstrapfarmer.com - no holes - by the 100 count box... 10 is a more reasonable starting number). Similarly with the inserts we've found that the cheap ones are basically use once.. maybe twice if you're careful.. three times if you don't mind them being half fallen apart, so we've gone to heavier duty ones.. Mostly we have ones made by "sunpack" which are pretty decent. For most flowers you'd want 6 cell (which fit 12/tray so 84 cells), larger stuff like melons and squash a 4 cell which fit 8 per 1020 tray (so 32). We start peppers and tomatoes in the 6 cell ones and then up-pot to a 4"widex6"deep pot once they're at second true leaves. The more robust inserts we're on year 3+ and they're all still good as new... less annoying and long term I think they pay for themselves.
For onions I made some cedar (old fence boards..) boxes about 5" deep with 1/4" mesh bottoms that just fit in the 1020 trays. I line bottom with landscape cloth and then I can plant 5 or 6 rows per box at about 1/2" spacing. These also work pretty well for flowers although it's a touch harder to pick those out (depending on the flower and how sensitive it is for root disturbance a lot are pretty hardy.. agastache and Amaranthaceae aka celosia aside..).
Light.. depending on the time of year the greenhouse might not be sufficient. For our early starts I have an indoor setup that's basically a rack that has 4 shelves and fits 4 1020 trays per shelf (so 16 trays total). Over each shelf I have 3 x 4' LED 2 bulb shop lights (6 bulbs covering the 20" depth) that are either 6500k or 6000k both seemed to work fine. I have those on a timer so they go on at 7:00AM and off at 10:00 PM. Plants like light.. this setup has helped us a lot. We started our squash (all 120+ of them) out in the greenhouse this year and they did fine.. the melons in the same place.. not so much they'd have liked more temperate temperature and also more light. Tomatoes and peppers also get a nice boost from longer light intervals. If you're trying to get an early start and have less than 10 hours of sunlight.. some supplemental light to extend the day is IMHO critical.. if you're planting after you have 10 hours of sunlight.. mostly fine.
Heat.. Greenhouses are a blessing and curse on this one. Plan for some thermal management. We've cooked more than a few things when they day got a bit hotter than expected and we didn't open the vents as much as we should have. They can get REALLY HOT even on a fairly cool day. Automatic venting would be really nice to have IMHO if the budget allows. I believe Darren has played with the wax filled cylinder vent activators? which are a no power option, I've heard mixed reviews on the longevity of some of the cheaper ones.
On the flip side some flowers and for sure peppers and tomatoes really benefit a LOT from extra bottom heat. A temperature controller and heat mat (or two.. depending on how much you're starting..) makes a huge difference. Having a heat mat easily tripled our pepper germination rates and doubled our tomatoes. We also use it for some warm soil loving flowers (look up the desired soil temperature and adjust accordingly..). There are some things it turns out this is a terrible idea for though - like melons.. do not use a heat mat for melons you'll cook the roots.. yes even on low...
Moisture is mostly watering enough.. but not to much. You want your potting mix moist but not wet, to wet and plants will damp off. If you can bottom water that's also helpful. Top watering tends to disturb the medium and also more risk of damping off. To dry is obvious ofc. We've had some luck with the tray covers to keep humidity up for things that like that.. and in some other cases we ended up with mildew and damping off.. My conclusion on those is use them early but sparingly and take them off once the seeds are up. As noted above bottom water when you can (again the decent trays are nice for this..) although sometimes you'll have to lightly water the top if it's not taking water up enough, some sort of mister is helpful for that.
Finally I'd say that seed starting is about as much art as science and specifics vary a bit depending on the plant... so keep at it I suppose.
We just filled 4 trays with 32 cells each for fall planting, it's time to start your fall cabbage right about now
It never ends!
