Updated at: 15-03-2023 - By: Sienna Lewis

Do you know what level of heat is best for petunias in a greenhouse? It’s not always obvious what to do because the rules change with each new stage of development. Nonetheless, many horticulture departments at universities are happy to share their knowledge about starting petunias from seed and nurturing them through their entire greenhouse growth cycle.

Only by using the greenhouse in the way that best suits each individual plant will it produce healthy, robust plants. When growing petunias in a greenhouse, temperature is one of the most important factors to control. When these conditions are met, you can enjoy the benefits of greenhouse gardening regardless of the weather outside.

What Are The Optimal Temperatures In A Greenhouse For Growing Petunias: Gardener’s Guide

Different petunia stages—planting, rooting, and growth—require varying temperatures, which are listed below. Depending on the climate where your plants thrive, you’ll need to make adjustments to the greenhouse’s environment. Read up on your hardiness zone to make sure you can plant these flowers successfully. The Missouri Botanical Garden recommended zones 10 and 11.

What Are The Optimal Temperatures In A Greenhouse For Growing Petunias - Krostrade

Optimal temperatures for planting petunias

Petunia seeds can be started in containers, but the ideal temperature for germination is between 75 and 80 degrees Fahrenheit, as stated by Iowa State University. Direct sunlight and temperatures above 90 degrees Fahrenheit can thwart the germination process. Temperatures of 60 to 65 degrees Fahrenheit are ideal once the seeds begin to germinate, which takes about 7 to 10 days.

In contrast to the more natural results achieved when planting petunias outdoors from transplants, greenhouses are ideal for those who wish to grow petunias from seed. However, when transferring petunias from a greenhouse to a garden, what temperatures should they be exposed to? Waiting until the soil temperature reaches 60 degrees Fahrenheit is suggested by the University of Minnesota Extension to prevent frost damage.

Moreover, before you transplant them, make sure the nighttime temperature drops to between 63 and 65 degrees Fahrenheit. According to research conducted by Utah State University, optimal soil temperatures for seed germination are between 65 and 75 degrees Fahrenheit (or more specifically, 70 and 72 degrees Fahrenheit) in a greenhouse. Petunias can be grown in a greenhouse at Auburn University with temperatures between 75 and 78 degrees Fahrenheit for the first five days, 68 and 75 degrees Fahrenheit once the cotyledons have unfurled, and 65 and 70 degrees Fahrenheit once the true leaves have appeared.

As for the water temperature in the greenhouse, aim for 70 degrees for irrigation and misting.

Optimal temperatures for rooting petunias

Rooting petunias is an alternative to growing them from seeds that some gardeners prefer. Rooting the plants for 2–3 weeks at 64–75 °F is ideal for this technique. Plants grown from cuttings or cuttings that have been rooted will look and bloom at the same time as the original plant, which is an advantage of this method.

Optimal temperatures for growing petunias

Keep in mind the day and nighttime temperatures for successful petunia cultivation and upkeep in the greenhouse. Daytime highs should reach the upper 60s to low 70s Fahrenheit, with corresponding lows of 55 to 64 degrees at night. Petunias are typically not heat-tolerant, but there are exceptions.

Did you know that petunia temperatures influence the plant’s final height, when it flowers, and whether or not it forms side branches? In a range from 50 to 77 degrees Fahrenheit, plants can grow taller, bloom sooner, and produce fewer side branches. Experts advise using 63°F for the first ten days after transplanting petunias.

Other Greenhouse Requirements For Growing Petunias

Petunias in the greenhouse have additional requirements besides temperature. Always go for a pH of 6.0 to 7.0 and a soil that is light and well-drained with average fertility. Also, a sheltered area is helpful for transitioning petunias from a greenhouse to outdoor beds.

Once your petunias are well-rooted, you can increase your watering frequency to once every week, providing as much as 2 inches. Blooming and growing can be promoted by fertilizing once a month with a balanced fertilizer. If your petunias stop blooming, you can revive them by cutting back the stems but leaving the leaves intact.

Flowers can be encouraged to bloom by using a liquid fertilizer and providing ample water. Deadheading refers to the process of removing spent blossoms from perennials. Usually, they will do this on their own if you use cultivars that produce smaller flowers.

Can you tell me what sorts of challenges you might face when trying to cultivate petunias? This flower has a few problems, like aphids and slugs, which is a good thing. Petal blight and other diseases can be prevented or treated with proper watering and humidity control.

What Is The Best Petunia For Greenhouse?

Petunias are a popular flower in gardens, with the most well-known varieties including the single-flower Grandiflora and Multiflora varieties, as well as their double-flower variants. Large flowers characterize the Grandiflora, and many cultivars offer a rainbow of hues; however, the more compact Multiflora can thrive in harsh environments.

Petunia Cold Tolerance

An overview of this flower’s cold resistance is as follows:

Petunias do best in temperatures between 61 and 80 degrees Fahrenheit during the day and 55 to 65 degrees Fahrenheit at night.

Even though Wave® petunias can survive temperatures as low as 35°F, they will quickly perish in temperatures below 40°F.

If petunias have been slowly acclimating to the cooler temperatures of autumn, they may be able to withstand a light frost or dusting of snow.

What Is the Growing Season for Petunias?

Favorites of the cottage garden and the hanging basket, they flourish in warm weather.

In climate zones that are not 9–11, petunias cannot be planted outside until the weather is consistently warm. This is true whether you start the seeds indoors eight to ten weeks before your average last frost date or buy the plants from a nursery.

It’s important to remember that some of the strategies that will help you postpone the negative effects of freezing at the end of the season won’t work in the beginning of the season, when the plants are still immature and more susceptible to the effects of temperature shock.

Only petunias that did well during the summer and then gradually adapted to the increasingly cold temperatures in the fall will reap the benefits of late-season efforts to keep them alive during the winter.

For instance, newly planted petunias that are exposed to early spring frosts in containers will not survive in a full-sun location or under fabric covers.

It is important to harden off plants before transferring them to the garden or permanently leaving them in containers outside.

This can be accomplished by exposing them to the outdoors for increasing amounts of time over the course of a week to ten days, starting with just one hour on the first day in a sheltered area.

If a cold spell is predicted, bring them inside and start over when the weather warms up again.

Most cultivars, when grown as annuals, start flowering around the middle of summer and keep going until the first hard frost.

Need to know how to maximize your flowering time? Read on.

Winter Care in Zones 9-11

Have fun with petunias as perennials when you live somewhere they can be grown. They won’t hibernate in the winter, so there’s no point in protecting them from the elements.

There are, however, a few helpful errands to run between November and March to ensure that your petunias thrive.

Keeping up with the plants’ watering needs, for instance, can encourage them to remain healthy and flowering even when temperatures are lower in the fall and winter.

Their ideal soil moisture is between six and eight inches deep, so they need supplemental water to achieve that.

Petunias planted in the ground may need an extra watering once or twice a week, but those kept in pots and hanging baskets may need watering every day if it hasn’t rained.

Remember to apply a new layer of mulch in the garden beds or large containers in October or November if you grow these delicate perennials year-round.

This will prevent moisture from evaporating and weeds from growing. Soil that is both healthy for plants and easy to work can be created with the help of organic matter, which breaks down over time.

You can use dry grass clippings, shredded leaves, or bark, or wood chips. Put down a layer of mulch about two inches thick, keeping it at least that far away from the trunks of any plants in the center. As a result, the shallow-rooted plants will be able to keep more moisture in their surroundings.

If you have plants in containers and haven’t fertilized them in at least six months, you should apply slow-release fertilizer granules in the fall.

Don’t go crazy with the nitrogen, or your plants will focus on growing leaves instead of flowers.

In addition, perform a critical plant inspection in early November and again in three months.

You can encourage new growth by “deadheading” or cutting the stems to half their height if you notice any withering flowers or straggly stalks.

If an unexpected ice storm or frost is predicted, be sure to cover any petunias that you can’t bring inside. That might not be enough to save them, but it’s a good bet.

The best coverings to use are made of fabric, such as burlap, as they prevent frost from getting to the plants but still allow moisture to evaporate.

Covers for rows can also be made out of sheets, pillowcases, or even thin tablecloths. Only make sure the bottoms of them are flat on the floor. Use rocks or other heavy objects to secure them.

When I needed to save some annuals in a pinch, I’d also use upended cardboard boxes as coverings. In a pinch, plastic can be used instead. The leaves and flowers can be frozen to death if it comes in contact with them because it traps moisture.

If you absolutely must use plastic, drape it over a tomato cage or some other improvised support so that it doesn’t touch the plants.

If you’ve covered your plants to keep them from getting too much direct sunlight, remember to take them off when the sun comes out or the temperature rises.

Optimal Temperatures In A Greenhouse For Petunias - Krostrade

Tips to Extend Blooming in Cooler Zones

Even if you don’t live in Zones 9–11, you can still extend the life of these annuals and even their blooming period. Implement these methods:

1. Variety Selection

Carefully choose which petunia variety to grow if you want to enjoy them until the first frost, and even after that.

If there is an unexpected early frost, it will be much more challenging to cover spreading petunias, for example.

Many multiflora varieties are smaller and easier to care for than their single-flora counterparts.

Choose a variety that can be brought inside for a day or two to survive the first frosts, or bring them inside for the winter, whether they are dormant or actively growing, if you are willing to grow them in containers.

The best way to keep petunias alive through the winter is to bring them inside, and we have a guide to help you do just that.

Choose a smaller, lighter variety, such as “Supertunia Mini Vista Pink Star,” for these containers. Home Depot sells them in packs of four.

Or, if you want to keep the weight of each hanging basket to a minimum when the weather cools, plant just one trailing petunia in each.

This also makes it easy to relocate the plants to cooler environments if the heat of midsummer starts to cause them to wilt.

One variety that can fill a basket by itself is Shock Wave® ‘Coconut. The flowers are a brilliant white, but their centers are yellow. Six-packs of these plants can be purchased from Burpee.

2. Location Selection

There are compromises in this section of the strategy. Plants need at least six hours of direct sunlight per day after being transplanted or sown.

However, you don’t need quite that much sunlight to cultivate these flowers. Part shade planting will extend the life of plants in regions with extremely hot summers, despite the fact that it reduces flowering.

Let me explain. Shaded areas tend to be cooler in the late autumn, which could cause the plant’s roots and stems to freeze and die more quickly than those in full sun.

An acceptable middle ground? Plant in full sun to protect from nighttime chills, but use shade netting to lessen the effects of the sun during the warmer months if it causes problems.

Avoid planting in valleys of your garden, where cold air can collect and cause frost.

Plant your annual petunias in a spot where the soil won’t dry out and the plants won’t be blown over.

3. Deadheading

Many of the newer varieties of spreading petunias, such as the Wave® series, have been developed to bloom nonstop without the need for deadheading.

However, by carefully removing faded blossoms from older varieties like grandifloras, you can extend the blooming season well into the fall.

The plan is to get rid of them before they start draining energy from the plant. When flowers wither, plants divert resources to seed production that could have been used to produce additional flowers.

Cut off the stems of the flowers where they are no longer blooming, using clean shears to prevent the spread of disease.

4. Pruning

While it may seem cruel at the time, midsummer pruning results in bushier, more floriferous plants come spring.

Do this task when you see the stems getting leggy and shedding leaves.

The drastic trim will stimulate new growth and another flush of blooms, and it will also strengthen the plants so they can withstand the coming cold.

5. Fertilizing

In order to ensure the best possible growth of these annuals in the garden, a balanced granular fertilizer should be worked into the soil about eight ounces per 25 square feet before planting.

Starting in the middle of summer, feed your petunias a dose of liquid fertilizer every three weeks.

When plants have covered a large area of the soil’s surface, making it difficult to scratch in the granules, it’s best to go with the liquid fertilizer.

Following the manufacturer’s recommendations, dilute the liquid and apply it to the soil or spray it on the leaves.

Add some granular, slow-acting fertilizer to the growing medium when you plant, and consider adding more midway through the growing season, if you want your hanging basket or container plants to flower for as long as possible.

This slow release of nutrients from the granules means that the fertilizer can remain in the container for a longer period of time. When growing annuals in containers, it is necessary to water them frequently, which can wash away any liquid fertilizer that has been applied.

Some people who are passionate about their hanging baskets will both add granules of fertilizer to the soil and spray liquid fertilizer on the foliage of their petunias on occasion.

This could be beneficial for your petunias if you water them daily or more frequently.

6. Providing Ample Space in Containers

When given sufficient room to expand while still being contained within a container, varieties that benefit from being grown indoors will bloom for significantly longer and be much less affected by cold weather.

Even though the negative effects of too-cramped quarters won’t become apparent until the plants get a bit larger and start competing for resources, it’s best to make sure of this space when you’re ready to transplant them.

If you plant too many in a container, the soil will dry out and the petunias won’t get the water they need.

Without easy access to water and nutrients, plants will wither and die in dry, compact soil.

Even though it may be tempting to fill a pot or basket with transplants for “instant color,” keep in mind that you only need a few plants to quickly reach full growth. Without crowding, a 10- to 12-inch pot can support two or three plants.

Plant small varieties 12 inches apart and larger ones 16 inches, if you’re using window boxes.

7. Temporary Coverings

Nature is unstoppable, and resisting her is futile. If you live in a region where petunias must be grown as annuals, and you intend to keep them outdoors, they will eventually perish due to the cold.

If you’ve been using these methods to keep your plants alive and blooming up until the first frost, row covers will help them survive the initial cold snaps.

A cage or frame should be used to elevate it so that it is perched just above the foliage and flowers; the edges should be weighted down using a brick or log.

Since only a small amount of water can penetrate the weave, you should remove it before the next rain.

It’s important to remember to uncover your plants once the weather warms up again, so that you can admire their vibrant blooms without risking overcooking them due to the cover’s trapped heat.

Using row covers or wire cages to grow your plants through the winter isn’t appealing, so you should save them for a later time.

Can Petunias Chill Out?

Whether you live in a zone where petunias are perennials or annuals, taking extra care of them as winter approaches has additional benefits.

If you put in the time and effort to assess your situation and perform any necessary additional maintenance, you can bring your garden and its environs closer to being a riot of color throughout the year, and not just in the flower beds and along the fence.

Potted plants, window boxes, and containers that serve as a bridge between your indoors and outdoors can all help you extend the flowering time of your outdoor plants.

Taking cuttings of your favorite petunia plants and rooting them so they can spend the winter in your home may also help them live longer. When spring comes around, you can move them outside to your garden or containers.

How To Prepare A Greenhouse Petunia For Planting - Krostrade

If you’re already planning to grow petunias in your garden, any advice you have for keeping them healthy in the fall would be greatly appreciated by other petunia enthusiasts. Feel free to post any questions, comments, or suggestions you may have below.

And if you’re interested in learning more about growing annuals after reading these season-ending tips, check out these best-selling flower books:

Optimal Greenhouse Humidity – Why ~80% Relative Humidity?

For successful crop production, relative humidity is an essential climatic factor. As a result, most growers strive to keep humidity levels where they should be for the plants they cultivate.

In general, most plants do best with a relative humidity of around 80%. It is at this point that typical greenhouse plants develop at their quickest rates. Plants’ physiological processes may be slowed by either increased or decreased humidity, resulting in slower growth and lower quality output.

Common humidity diseases, like botrytis or powdery mildew, are also greatly exacerbated by high relative humidity levels.

Growers shouldn’t just try to keep humidity levels as low as possible, but rather should learn about, control, and maintain humidity levels in accordance with their intended crops.

Optimal Greenhouse Temperature – Why 18°C (64°F)?

Most crops grown in greenhouses thrive between the temperatures of 18 and 24 degrees Celsius (64 and 75 degrees Fahrenheit).

These are the typical greenhouse temperatures because they are thought to be ideal for the majority of crops. Growers rarely deviate from this optimal temperature range because temperatures outside of it typically result in slowed or halted growth and suboptimal crop quality.

Humidity regulation is essential in controlled environments such as greenhouses and indoor farms. Because high humidity causes so many issues and wastes so much time and resources in these kinds of environments if it is not controlled. A crop’s growth rate, size, and quality are all negatively impacted by less-than-ideal environmental conditions.

Humidity that is not controlled will lead to condensation within any building. Diseases like botrytis and downy mildew thrive in the presence of this free water and can quickly wipe out entire fields of vegetables, cannabis, or any other crop.

Example of Water Extraction Rate Graph

See how fast the new DG-12 EU unit can draw water at different temperatures (from 64 to 75 degrees Fahrenheit) in the graph below.

As the temperature and relative humidity increase, so does the rate of extraction.

The device drains 48 liters of water per hour under ideal greenhouse conditions of 80% relative humidity and 18oC. There is a noticeable increase in the rate of water extraction at higher temperatures.

DryGair Provides Optimal Humidity and Temperature for Common Greenhouse Conditions

DryGair is one of the few dehumidifier producers who zero in on the specific needs of the farming industry. At 18oC and 80% relative humidity, which are typical growing conditions, the units effectively reduce humidity. As a result, DryGair can extract over 4 liters (1 gallon) of air per kWh of energy input.

Greenhouses and indoor cultivation can benefit from DryGair units running during the following problematic times:

  • Situations where heat losses from heating and ventilating are unavoidable
  • From nightfall until morning light (learn more about a typical and constant climate)
  • When the relative humidity outside is greater than the relative humidity inside (i.e., during rainy or humid weather), we say that it is “whenever.”

Condensation at the dewpoint, brought on by sudden increases in humidity, has been linked to the spread of many diseases. These peaks are most common at dawn and dusk, when environmental conditions undergo rapid shifts, and can be avoided with DryGair’s humidity control solution.

DryGair is a tool for controlling humidity in greenhouses, with a high rate of water extraction:

  • At 18 degrees Celsius (64 degrees Fahrenheit) and 80% relative humidity, it can hold up to 48 liters per hour (13 gallons).
  • In total, 576 liters (152 US gallons) are drained from the system overnight.

All of DryGair’s performance metrics are recorded at a temperature of 18C (64F) and a relative humidity of 80%. With an increase in both temperature and humidity, the rate of water extraction increases.

The Temperature Range for Petunias

Many amateur gardeners opt for Petunias (Petunia spp.) because of their colorful flowers and wide range of available sizes and shapes. These low-maintenance plants provide homeowners with a wide range of options, including trailing, spreading, and fragrant cultivars that will bloom continuously throughout the summer. Petunias prefer full sun.


Petunias are only perennial herbs in USDA plant hardiness zones 10 and 11. When the daytime temperature stays between 70 and 85 degrees Fahrenheit, the flower seeds will begin to germinate. According to the University of Minnesota Extension, once they are established, they thrive in temperatures that do not fall below 55 degrees at night and 65 degrees during the day. Even so, petunias are one of the most popular bedding plant options because they can be grown as annuals in cooler USDA zones.


Petunias enjoy lots of light and warmth. It is best to avoid putting them in the shade, as they require full sunlight to flourish. When summer’s heat is particularly intense, the flowers may wilt. If your flowers are wilting in temperatures over 85 degrees for an extended period of time, you should prune them in half and feed them a balanced liquid fertilizer formulated for flowers.

Soil & Water

Your flowers will not flourish in any environment, regardless of the temperature. Soil should be consistently moist, but never soggy, for petunias to flourish. Petunias should be planted in a raised bed or in containers like hanging baskets if water collects in your garden. The bases of containers should have drain holes. The ideal soil has a pH between 6.0 and 7.0 and is light and loamy.

Other Considerations

To encourage further blooming, you should deadhead certain varieties of petunia by removing spent flowers. Some flowers naturally lose their petals. If slugs are a problem where you live, use slug bait to protect your plants from being eaten. If it’s humid and wet outside, blight could affect them as well. Avoid getting water on the foliage and blossoms by watering at the soil level.


There are many articles written by university extension services about the benefits of growing and harvesting petunias in a greenhouse. But are you aware of the ideal greenhouse temperatures for cultivating petunias? A major factor in petunias’ growth and development is the greenhouse’s temperature, so this is an important query to consider.

There are distinct temperature needs at each of the three stages of plant development (planting, rooting, and growth). Petunias’ germination, growth, and quality can be negatively impacted if these conditions aren’t met. Petunias thrive in temperatures between 64 and 75 degrees Fahrenheit during the day and 55 and 64 degrees Fahrenheit at night in a greenhouse. Knowing the average annual temperature range for your planting zone will also be helpful.