Growing peppers in a greenhouse

Peppers are the crop a greenhouse improves most dramatically, and the reason is at both ends of the season. They germinate slowly and reluctantly in cold media, which makes an early start difficult without bottom heat, and they need a further two to four weeks beyond the green stage to color, which most short outdoor seasons never provide. Under cover both problems disappear.

They are also the crop most often started too late. A pepper sown at the same time as a tomato will be four weeks behind it at transplanting, because germination alone takes 8 days against 6 and early growth is much slower. Sowing peppers eight to ten weeks before transplant, rather than the six that suits tomatoes, is the single most useful scheduling change.

How do you start peppers under cover?

Sow into cells at 80 to 85 degrees Fahrenheit in the media. This is not optional in the way it is for tomatoes: below 65 degrees pepper germination becomes erratic enough that half a tray failing is normal. Use a VIVOSUN 10 x 20.75 in Seedling Heat Mat and Digital Thermostat Set and set the probe into the media of the tray furthest from the mat center, because the temperature across a mat can vary by 8 degrees.

Expect 8 to 14 days even at the correct temperature, and considerably longer for hot types than for sweet ones. Do not conclude a tray has failed at day 10. If you are working from seed more than two years old, run a germination test on blotter sheets first rather than committing bench space and three weeks to it.

Drop the media to 70 to 75 degrees by day and 65 at night once seedlings are up, and keep the mat off. Peppers grow slowly at this stage and that is normal: a pepper seedling at four weeks looks like a tomato seedling at two. Resist the urge to feed heavily to speed it up, because soft growth transplants badly.

Pot on into 3 to 4 inch pots once the first true leaves are established, and keep them growing without check. Peppers respond badly to being pot-bound, and a plant that stalls in a small pot often flowers prematurely, sets one fruit and then stops, which wastes the whole season.

What temperature does peppers need in a greenhouse?

Established plants want 70 to 82 degrees Fahrenheit by day and 62 to 68 by night, which is warmer at both ends than tomatoes. The night figure is the one that limits an unheated greenhouse: below about 58 degrees growth effectively pauses, and the plant sits without dying for as long as the cool spell lasts.

The heat ceiling is the same 95 degrees as tomatoes and for the same reason, non-viable pollen. Peppers show it slightly differently: rather than dropping flowers cleanly, they often set small misshapen fruit that never sizes up. If a plant is producing many tiny distorted peppers in high summer, the cause is temperature at set rather than nutrition.

Peppers also suffer sunscald, a pale papery patch on the exposed shoulder of a fruit, which is genuine tissue damage from direct sun at high temperature. Unlike tomatoes, where leaf stripping is beneficial, peppers need their leaf cover kept over the fruit. Combine that with 50 percent shade cloth through the hottest weeks.

A useful practical detail: peppers tolerate a smaller day to night difference than tomatoes and actually prefer a warm night. Where a tomato wants a 10 to 15 degree drop, a pepper does best with 8 to 12, and dropping the night setpoint too far slows it noticeably.

How much light does peppers need?

Peppers want 20 to 30 moles per square meter per day and stall below about 15. That is slightly less demanding than tomatoes and still well above what a winter greenhouse provides, so the same conclusion applies: peppers are a spring-to-autumn greenhouse crop unless you are lighting them seriously.

The place supplemental light pays is the propagation stage. Pepper transplants raised on a windowsill at a daily light integral of 3 or 4 come out tall and weak, and they never catch up. A rack with two 2 ft strips per shelf, held two to four inches above the canopy and raised weekly, produces a stocky plant that transplants without check.

Because peppers take eight to ten weeks from sowing to transplant, that lighting period is long, and it overlaps almost entirely with the darkest weeks of the year at mid latitudes. Budget for it as part of the pepper plan rather than as an afterthought.

Spacing, support and training

Space at 14 to 18 inches in rows 24 to 30 inches apart, giving 2.5 to 3.5 square feet per plant. Peppers are more compact than tomatoes and tolerate closer spacing, but the same airflow logic applies: a dense pepper canopy in still air holds humidity and invites bacterial spot and botrytis at the fruit stalk.

Support is where peppers differ most from tomatoes. They do not train to a single leader well: the plant branches naturally at a Y fork, and a heavy crop splits that fork, which usually ends the plant. String training a pepper is therefore the wrong technique. What works is a cage or a ring of stakes fitted when the first fruit sets, and clip-on stake arms convert a single stake into a cage at exactly that point.

Removing the first flower, often called the king flower, that forms at the initial fork is worth doing. Left in place it takes priority and holds the plant back for two to three weeks; removed, the plant puts that energy into structure and then sets far more fruit overall. This is standard commercial practice and it feels wrong the first time you do it.

Prune the growth below the first fork entirely. Those lower shoots produce nothing useful, block airflow at the base of the plant and hold moisture against the stem, which is where the plant is most vulnerable.

Watering and feeding

Peppers use less water than tomatoes, roughly 0.25 to 0.3 gallons per square foot per day at peak, and they are less forgiving of overwatering. Waterlogged pepper roots stop functioning quickly and the visible symptom, wilting, looks identical to drought, which leads people to water more and kill the plant faster.

Check the medium rather than the leaves. A bed with a top inch that is dry is usually fine; a pot that feels heavy and looks wilted is drowning. In containers, fabric grow bags help substantially because they breathe through the walls and are physically difficult to overwater.

Feed lightly with balanced nutrition until the first fruit sets, then move to higher potassium. Peppers are more sensitive to excess nitrogen than tomatoes: an overfed pepper produces a large dark green plant with almost no fruit, and it will stay that way for the rest of the season.

Calcium matters here too. Peppers get blossom end rot for the same reason tomatoes do, irregular water rather than soil calcium, and the fix is the same: consistent delivery from a timer rather than a can.

What goes wrong with greenhouse peppers?

Which types work best under cover?

Harvesting and what comes next

Pick green fruit to keep the plant producing, or leave fruit to color for flavor and vitamin content. You cannot fully have both: every fruit left to color suppresses further set on that plant, and a plant carrying six ripening peppers stops setting new ones almost entirely.

The practical compromise on a small number of plants is to pick the first flush green, which pushes the plant into a second heavy set, then allow the second flush to color as the season winds down. That produces more total weight than either strategy alone.

Cut rather than pull. Pepper stems are brittle at the fruit joint and pulling frequently takes a whole branch with it, which on a plant with four productive branches is a quarter of the crop gone.

Check the germination figures against the germination soil temperature chart, plan the long lead time with the frost date sowing calculator, and read the pollination detail in pollination in a greenhouse.

Working out the figures for your own house and season? The Greenhouse Build & Growing Planner is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.