Pelargoniums are the classic greenhouse ornamental, and the reason is entirely practical: they are tender enough to be killed by the first frost outdoors and tough enough to survive a whole winter under cover on almost no water and almost no attention. A greenhouse turns an annual bedding plant into a permanent one.
They are also among the easiest plants to propagate. A cutting taken in late summer roots in a few weeks and flowers the following season, which means one plant becomes a bench of them without buying anything. For a greenhouse used mainly for vegetables, pelargoniums are the ornamental that costs the least to keep and gives the most back.
| Measure | Figure | Why it matters |
|---|---|---|
| Germination soil temperature | 70 to 75 degF | Seed-raised types germinate readily at this temperature, though most garden geraniums are grown from cuttings instead. |
| Days to emergence | 7 to 14 days | At the optimum soil temperature, in a moist starting medium. |
| Optimum day temperature | 65 to 80 degF | Where growth is fastest. Above the range, specific processes start failing. |
| Optimum night temperature | 55 to 65 degF | The day to night difference controls stem elongation and plant habit. |
| Survives down to | 35 degF | Brief exposure on an acclimated plant, not a sustained figure. |
| Trouble above | 90 degF | Flowering slows in sustained heat and the plant becomes leggy with sparse bloom. |
| Target daily light integral | 12 to 20 mol/m2/day | Moles per square meter per day. Below this the crop holds rather than grows. |
| PPFD needed at 16 hours | 208 to 347 umol/m2/s | What a fixture must deliver at the canopy to reach that daily total. |
| Spacing under cover | 10 to 14 in each way | Wider than outdoor spacing, because still air makes disease the limit. |
| Floor area per plant | 0.7 to 1.2 sq ft | Includes each plant share of shoulder and path space. |
| Sowing depth | 0.125 in for seed, cuttings at the first node | Seed is small and needs light contact with moist medium. Cuttings root from the node, so that is what goes below the surface. |
| Soil pH | 6.0 to 6.8 | Outside this band, nutrient availability rather than nutrient supply becomes the limit. |
| Days to first harvest | 12 to 16 weeks from seed to flower, 8 to 10 from a cutting | From sowing, under cover, at good light. Winter timings run considerably longer. |
| Humidity preference | 0.8 to 1.2 kPa VPD | Expressed as vapor pressure deficit, which accounts for temperature as relative humidity does not. |
How do you start geraniums under cover?
Take cuttings rather than sowing seed for named varieties, because seed does not come true. Cut a non-flowering shoot tip about four inches long, trim it cleanly just below a leaf node, remove the lower leaves and any flower buds, and insert it into a free-draining medium.
Do not use a humidity dome for pelargonium cuttings. This is the opposite of standard cutting practice and it matters: pelargonium stems are succulent and rot readily in saturated air. They root perfectly well in ordinary greenhouse conditions with the cut end in gritty medium, and covering them is the usual reason a batch fails.
Late summer is the traditional timing, because it takes cuttings from growth that is still soft while leaving time for roots to form before winter. Rooting takes two to four weeks in warm conditions.
For seed-raised bedding types, sow at 70 to 75 degrees Fahrenheit in a standard tray 12 to 16 weeks before you want flowers. Seed-raised pelargoniums flower reliably in their first year, which is why they dominate commercial bedding production, but the flower quality on named cutting-raised varieties is generally better.
What temperature does geraniums need in a greenhouse?
Pelargoniums want 65 to 80 degrees Fahrenheit by day and 55 to 65 at night in the growing season, which is comfortable greenhouse conditions and not demanding.
The important number is the winter minimum, and it is lower than most people expect. Held nearly dry, pelargoniums survive at 45 degrees Fahrenheit and tolerate brief drops toward 35. That is a frost-free house rather than a heated one, which puts overwintering within reach of a modestly heated greenhouse or even an unheated one in a mild climate with some thermal mass.
Dryness is what makes the low minimum survivable. A pelargonium held at 45 degrees in dry medium is dormant and untroubled; the same plant at the same temperature in wet medium develops blackleg and rots at the base. Cold alone does not kill these plants; cold and wet together do.
At the warm end, sustained heat above about 90 degrees produces leggy growth with sparse flowering. Shading in high summer keeps the plants compact and flowering, and pelargoniums want only 12 to 20 mol/m2/day so a 30 to 50 percent cloth costs nothing.
How much light does geraniums need?
Pelargoniums want 12 to 20 moles per square meter per day, which is 208 to 347 micromoles per square meter per second over a 16 hour photoperiod. That is a moderate requirement and a greenhouse supplies it easily through the growing season.
Light drives flowering directly. A plant in insufficient light produces long pale stems with widely spaced leaves and very few flowers, which is the classic appearance of a pelargonium overwintered in a dim corner and never cut back.
In winter the light requirement is not met in most greenhouses, and that is fine because the plant is dormant rather than growing. The correct response is to accept the pause, keep the plant dry and cool, and cut it back hard in late winter so the new growth that comes with returning light is compact rather than drawn.
Spacing, support and training
Space at 10 to 14 inches each way in active growth, which is roughly a square foot per plant. Overwintering plants can be packed far tighter, since they are dormant, leafless and not competing for light.
Airflow around the base of the plant is the main spacing consideration. Botrytis on pelargoniums establishes on dead flowers and lower leaves in still humid air and then moves into the stem, so anything that keeps the base open and dry pays for itself.
Pinch young plants at three or four leaf pairs to force branching. An unpinched pelargonium grows as a single stem with flowers at the top, and one pinch at the right moment produces a plant with three or four flowering stems instead.
Deadhead continuously. Spent pelargonium flowers hold moisture, decay in place and are the primary entry point for botrytis under cover. Snapping off the whole flower stalk at its base, rather than pulling the petals, is both tidier and more effective.
Watering and feeding
In the growing season, water thoroughly and then let the medium dry appreciably before watering again. Pelargoniums are semi-succulent and far more tolerant of drought than of saturation, and the most common growing-season error is watering on a schedule rather than on need.
In winter, water almost not at all. A dormant plant at 45 degrees Fahrenheit uses a negligible amount, and the medium should be kept just short of bone dry. A splash once every three or four weeks is more than enough, and more than that causes blackleg.
Water at the base and never over the foliage. Pelargonium leaves hold water at the junction with the stem, and that water sitting overnight in a cool house is where rot begins.
Feed with a balanced feed during active growth and switch to a higher potassium feed once flowering begins, in the same pattern as fruiting vegetables. Stop feeding entirely from late summer, so the plant hardens off rather than entering winter with soft growth.
What goes wrong with greenhouse geraniums?
| What you see | Usual cause | What to do |
|---|---|---|
| Stem turns black at the base and the plant collapses | Blackleg, a fungal stem rot from wet medium in cool conditions | Keep overwintering plants nearly dry. There is no cure once established, so take cuttings from healthy tips and discard the plant. |
| Grey fuzzy mould on flowers and lower leaves | Botrytis, establishing on dead tissue in still humid air | Deadhead continuously, remove yellowing leaves, widen spacing and run a circulation fan. |
| Long bare stems with leaves and flowers only at the tips | Insufficient light, no pinching, or a plant that was never cut back after overwintering | Cut back hard in late winter, pinch young growth at three or four leaf pairs, and increase light. |
| Leaves yellow from the bottom of the plant upward | Overwatering more often than nutrient shortage, particularly in cool conditions | Let the medium dry appreciably between waterings before adding any feed. |
| Rusty brown rings with powdery spots on leaf undersides | Pelargonium rust, a specific disease of zonal types spread by wet foliage | Remove affected leaves, keep water off the foliage, increase airflow, and destroy badly affected plants. |
| Small white insects rising when the plant is disturbed | Whitefly, which overwinters happily on an evergreen host in a greenhouse | Yellow sticky traps, biological control, and a period without susceptible host plants. |
| Cuttings rot at the base instead of rooting | Too much humidity, usually from covering them, or medium that holds too much water | Do not use a humidity dome on pelargonium cuttings. Use gritty free-draining medium and let the cut end callus first. |
| Few flowers despite healthy vigorous leaves | Too much nitrogen, or the plant was never given a cool dry rest | Switch to a higher potassium feed at flowering and give a genuine cool dormant period in winter. |
Which types work best under cover?
| Type | Suits | Note |
|---|---|---|
| Zonal pelargoniums | General bedding and pot culture | The classic type with a dark horseshoe marking on the leaf, upright habit and continuous flowering. |
| Ivy-leaved pelargoniums | Hanging baskets and edges | Trailing habit with glossy leaves, and generally more heat and drought tolerant than zonals. |
| Regal or Martha Washington types | Greenhouse display specifically | Larger showier flowers over a shorter season, and they genuinely need a cool winter rest to flower well. |
| Scented-leaf pelargoniums | Foliage rather than flower | Grown for leaf fragrance, very easy from cuttings, and among the most tolerant of neglect. |
| Seed-raised bedding types | Large numbers at low cost | Flower in their first year from a late winter sowing, which is why commercial bedding uses them. |
| Stellar and fancy-leaved types | Collectors and mixed displays | Distinctive leaf shapes and variegation, usually less vigorous and worth a sheltered spot on the bench. |
For a greenhouse used mainly for vegetables, scented-leaf types are the most rewarding pelargoniums to keep. They root from cuttings at almost any time, tolerate erratic watering, take up little space and give something back every time you brush past them.
Regal types are the ones that most need the greenhouse, because they require a genuinely cool winter rest to flower well and they are the most damaged by weather outdoors. They are also the most susceptible to whitefly, which is worth knowing before adding them to a house that already has a population.
Harvesting and what comes next
Deadhead continuously through the flowering season, snapping the whole flower stalk off at its base rather than pulling the petals. Spent flowers left in place are the primary entry point for botrytis in a humid house.
Take cuttings in late summer, before the plants are cut back for winter. This is the moment when there is plenty of suitable soft growth and still enough warmth for rooting, and it means you always have young plants in reserve regardless of how the overwintering goes.
Cut plants back hard in autumn as they go into dormancy, removing roughly half to two thirds of the top growth along with every leaf that is yellowing. A plant carrying a lot of soft foliage into a cool winter house is a botrytis problem waiting to happen.
Cut back again in late winter, harder if the plant has become leggy, just as light begins to return. The growth that follows is compact and flowers earlier than growth from an uncut plant.
Repot into fresh medium in late winter at the same time. A pelargonium that has spent a year in the same pot is in exhausted medium, and fresh medium at the point of cutting back is what produces a strong season.
Equipment for growing geraniums under cover
Pelargoniums are the plant that most rewards a frost-free greenhouse and asks the least of it. Cut them back in autumn, keep them cool and nearly dry through winter, cut them back again and repot in late winter, and take cuttings every late summer so you always have replacements. Done that way a single plant lasts for years and multiplies for nothing.