Growing tomatoes in a greenhouse

Tomatoes are the crop most hobby greenhouses are bought for, and they are also the crop with the narrowest working range of anything you are likely to grow. The plant itself is tough: it will survive from just above freezing to well over 100 degrees Fahrenheit. What is fragile is fruit set, which depends on pollen that is viable across a band of only about 40 degrees.

That single fact reorganizes everything else. A greenhouse tomato that grows enormous foliage and sets nothing is not underfed, it is experiencing temperatures outside the pollen window, usually at night in spring or at midday in summer. The instruments that matter most for tomatoes are therefore a max-min thermometer and a shade cloth, not a fertilizer.

How do you start tomatoes under cover?

Sow about six weeks before you intend to transplant, and time the transplant so that the greenhouse night minimum is reliably above 55 degrees Fahrenheit by then. Working backward from that figure rather than from a calendar date is the whole trick: a heated house lets you sow far earlier, and an unheated one does not, regardless of what the packet says.

Start in cells or blocks at 75 to 80 degrees Fahrenheit in the media, which almost always needs bottom heat. A VIVOSUN 10 x 20.75 in Seedling Heat Mat and Digital Thermostat Set holds that setpoint rather than simply running warm, and the distinction matters because an unregulated mat commonly overshoots into the mid eighties, which is fine for tomatoes but ruinous for anything else on the same bench.

Take the mat off within two days of emergence and drop the growing-on temperature to 65 to 70 degrees by day and 60 to 62 at night. This is the point at which most home-raised tomato transplants go wrong: held warm after germination, they elongate faster than they thicken, and the resulting tall, thin plant never fully recovers its vigour.

What temperature does tomatoes need in a greenhouse?

The working range is 70 to 80 degrees Fahrenheit by day and 60 to 65 by night, and both ends of both figures matter. Below 55 degrees at night, pollen release fails and flowers abort. Above 95 by day, pollen is denatured and the same thing happens. Between roughly 55 and 60 at night the plant sets fruit but sets it slowly and often with poor shape.

The night figure is the one hobby growers miss, because a greenhouse that reaches 85 degrees at noon feels like a warm greenhouse even when it falls to 48 degrees before dawn. The plant responds to the minimum. Two 55 gallon water barrels along a sunlit wall store roughly 920 BTU per degree Fahrenheit between them and typically lift the pre-dawn minimum by several degrees at no running cost, which is often the difference between setting and not.

At the other end, once daytime peaks routinely exceed 90 degrees the answer is shade before ventilation. Fifty percent shade cloth over the roof typically drops peak house temperature by 10 to 15 degrees, which is more than most hobby exhaust fans achieve. Tomatoes want a daily light integral of 22 to 30 and a summer greenhouse delivers 30 to 41, so cutting light by half costs nothing and saves the crop.

A deliberate day to night difference of 10 to 15 degrees Fahrenheit produces shorter, sturdier plants with better fruit set than a flat setpoint. A controller with separate day and night targets is the mechanism; on a manual heater, setting the thermostat down in the evening achieves the same thing.

How much light does tomatoes need?

Tomatoes want 22 to 30 moles per square meter per day and stall below about 18, which is among the highest requirements of anything grown under cover. In practical terms this makes tomatoes a spring-through-autumn crop in a hobby greenhouse at mid latitudes, and it makes winter tomato production a lighting project rather than a heating one.

Over a 16 hour photoperiod, 22 mol/m2/day requires a sustained 382 micromoles per square meter per second at the canopy, which is far more than a seed-starting strip delivers. Fixtures such as linked 4 ft LED strips are correct for raising transplants and for winter greens, and they are not a route to winter tomatoes. Check what a fixture actually delivers with the DLI calculator before assuming otherwise.

The exception is stretching the shoulders. Six weeks of supplemental light over transplants in late winter produces sturdy plants ready to go into the house the moment the night minimum allows, which brings first harvest forward by two to three weeks. That is a much better use of a light budget than trying to fruit through midwinter.

Spacing, support and training

Space at 16 to 20 inches in rows 30 to 36 inches apart, giving 4 to 6 square feet per plant. That is roughly twice the spacing a seed packet suggests for outdoor growing, and the reason is airflow rather than light: still greenhouse air keeps leaf surfaces wet, and a dense tomato canopy will grow beautifully for six weeks and then go down to botrytis.

String training is the single change that most increases yield per square foot. Run heavy polypropylene twine from an overhead wire down to each plant, attach the stem every 12 inches with trellis clips rather than ties, and remove every side shoot weekly. A single leader trained this way occupies 4 to 6 square feet instead of the 9 to 12 a sprawling plant takes, and it fruits along its full height.

Side shooting is not optional on indeterminate varieties under cover. Every shoot left in the leaf axil becomes a second leader, doubles the leaf area, halves the airflow and delays fruit. Do it weekly with fingers rather than secateurs while the shoots are under two inches, because a clean pinch heals faster than a cut.

Remove lower leaves progressively as trusses ripen, up to the level of the lowest ripening fruit. This is the second most useful cultural practice after side shooting: it opens the base of the canopy to airflow, which is exactly where botrytis starts, and it directs the plant toward fruit rather than foliage.

Watering and feeding

Tomatoes are the highest water user in a hobby greenhouse, at roughly 0.4 gallons per square foot of growing area per day at peak. More importantly they are intolerant of irregular supply: blossom end rot, the sunken dark patch at the base of a fruit, is a calcium transport failure caused by inconsistent watering rather than by a calcium shortage in the soil. Adding calcium rarely fixes it and consistent watering usually does.

That makes a timer and a drip line the most valuable single upgrade for a tomato crop. A Melnor AquaTimer 4-Zone Programmable Digital Timer feeding a quick-connect drip kit delivers the same volume every day whether or not you are there, and it keeps water off the foliage entirely, which removes most of the leaf wetness that drives disease.

Feed with a balanced feed until the first truss sets, then switch to a high potassium feed for the rest of the season. Nitrogen after fruit set produces leaf at the expense of fruit, which is the most common feeding mistake. In a bed rather than a container, feed less than the label says: greenhouse beds accumulate nutrients because no rain leaches them.

Water in the morning, not the evening. Water applied late sits on the surface all night, raises overnight humidity and puts the house into the dawn condensation condition described in the humidity and VPD chart.

What goes wrong with greenhouse tomatoes?

Which types work best under cover?

If you grow tomatoes in the same greenhouse bed each year, soilborne problems accumulate in a way they do not outdoors, because nothing is rotated and nothing is leached. Grafting is the commercial answer, and it is entirely doable at home with grafting film and a humid chamber for the week after the graft. The alternative is growing in fabric grow bags with fresh medium each season, which sidesteps the problem entirely at the cost of more watering.

Harvesting and what comes next

Pick at the point the fruit changes color rather than waiting for full ripeness on the plant. Tomatoes produce their own ethylene and ripen perfectly well off the vine once they reach the breaker stage, and picking at that point reduces splitting and takes load off the plant so the next truss fills faster.

Never refrigerate a tomato you intend to eat for flavor. Below about 55 degrees Fahrenheit the volatile compounds that produce tomato flavor degrade and do not return on warming, which is why a supermarket tomato tastes of nothing. This is one of the few genuinely well-established results in produce quality research.

At the end of the season, cut plants off at the base and leave the roots in place rather than pulling them, which avoids disturbing the bed structure. Remove all foliage from the house: leaf debris left on the floor is where botrytis and leaf mold overwinter, and the same bed will start next season with the same problem otherwise.

Get the pollination detail in pollination in a greenhouse, the spacing arithmetic in the plant spacing chart, and the sowing timing in the frost date sowing calculator. If temperature control is the limiting factor, start with the heater BTU calculator.

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.