What’s the Best Time to Plant Annuals?

Best time to plant annuals outside

We often get asked when the best time to plant annuals outside is. The answer varies a lot depending on what kind of spring we’re having, where you are located and how you are planting your annuals.

Whether you are planting in your garden, a flower bed, or doing some container gardening, choosing when the best time to plant your annuals is an important consideration. Planting annuals too early have a higher risk of being damaged or killed by cold snaps.

By definition, an annual grows for only one growing season. For example, a tomato plant would be considered an annual, as it grows all summer long and then dies in the fall when it gets colder in most areas.

One way to get the most out of that one growing season is being smart about when you choose to plant your annuals. Timing can be a crucial factor when it comes to getting your annuals started off right!

We put together this guide to discuss some important factors to consider when choosing when to plant your annuals.

The best time to plant annuals outside

There are a number of factors to consider when deciding when to plant, but you can always use your growing zone’s average last frost date as a general rule of thumb.

For Pennsylvania, planting outside shortly around Mother’s Day (second Sunday in May) is the best time to plant annuals. This will ensure that the annual is unlikely to be wiped out by a late frost.

Planting shortly after Mother’s Day also gives you a full growing season, allowing the plant to mature and grow to full size for a full summer of beauty. Planting vegetables early can also lead to an early harvest.

If you are starting your annuals from seed, you can time out when your seedlings will be sprouted and ready to plant so that they are ready to go around Mother’s Day. Check your seed packet’s germination time to do this.

If you have a greenhouse or some other protection from early spring cold nights, annuals can be planted earlier than Mother’s Day.

5 Things to Consider When Choosing When to Plant Annuals

If you are really getting eager to plant your annuals, there a number of factors that you can consider to decide if you can safely plant your annuals earlier than Mother’s Day, including:

  1. What growing zonal area you are planting in annuals in
  2. Whether the annuals are hardened off
  3. The type of annual you are planting
  4. The size of the plants
  5. Where you are planting the annuals (or what you are planting them in)

Overwhelmed? We’ll break down each of these factors below!

Consider your growing zone

1. Considering your growing zone

For most annuals, frost will kill or cause the plants to struggle significantly, so it’s important to determine what growing zone you are in.

The United States is broken up into 11 different growing zones, each with general growing conditions that can help you as you plan your garden.

What growing zonal area are you in?

Not sure what growing area you are in?

If you are in Ephrata, PA, where Homestead Gardens is located, your growing zone is 6. For growing zone 6 the last frost happens on average sometime between April 20th-May 20th. For this reason, we normally recommend that you wait until after Mother’s Day to plant your annuals outside.

To find the growing zone for your specific zip code, here’s a helpful tool to determine your growing zone.

When to to plant your annuals by zonal area

Below is a guide for when the average last frost date is for popular growing zones in the United States.

This list can be used as a reference for when the average last frosts are, but please bare in mind that these are only estimates, and a later than normal frost is always a possibility.

Average last frost date for popular United State growing zones

  • Zone 3: May 20th-June 20th
  • Zone 4: May 15th-June 15th
  • Zone 5: April 15th-May 15th
  • Zone 6: April 20th-May 20th
  • Zone 7: April 15th-May 15th
  • Zone 8: March 15th-April 15th
  • Zone 9: February 15th-March 15th
  • Zone 10: January 15th-February 15th
  • Zone 11: Rarely-January 15th

For choosing when to plant specific annuals for the best harvest or display, this growing calendar can be used.

Are your annuals hardened off

2. Are your annuals hardened off?

Typically, a plant that is hardy and hardened off will grow better once planted in another environment.

Growing centers and greenhouses don’t all harden off their plants before selling, so it can be something to ask them when shopping for your plants.

What does it mean to “harden off” plants?

Hardening off plants is the process of gradually exposing plants to harsher conditions, with the end result being a plant that is hardy, healthy, and well-adapted to be planted in a new environment.

One strategy of hardening off plants is to expose the plant to slightly colder temperatures until it’s strong enough and hardy enough to be planted outside.

Many times greenhouses will harden off plants before selling them, but not all greenhouses do this. To do this, they may expose the plants to harsh conditions while they are young so that when they are big enough to sell, the transition from the greenhouse to your home isn’t as abrupt.

It’s important to keep hardening off in mind when you are planting your plants early. It’s likely that most plants will grow better if they aren’t experiencing extreme shock in their new environment. 

Whether you are planting them in landscape, planters, or hanging baskets, it’s always a good idea to make the transition from cozy, warm greenhouses to your home a gradual one.

Plants that are hardened off will be much better apt to withstand an early planting (although, a hard frost or cold snap will normally wipe out even a hardy plant, depending on the plant).

Here’s how to make the transition to your home gradual 

One way you can harden off your plants yourself is to keep the plants in their pots that you bought them with, and place these pots in mostly sun during the day outside, and then during the night when temperatures drop, taking them into your garage or home.

This allows the plants to be acclimated to your environment. Ultimately, a plant that doesn’t have as much shock and is more hardy will take off and grow better in your garden!

What type of annual are you planting

3. What type of annual are you planting?

Another thing to consider is what type of annual you are planting. 

Different varieties of annuals have different levels of hardiness. For examples, pansies and osteospermums are generally more resistant to cooler temperatures (temperatures around freezing), but plants such as vinca and zinnias are much more sensitive to cooler temperatures. 

If you’re getting spring fever early before most flowers are ready to plant out, one option for you is planting some cold resistant plants early and then replacing or supplementing the plants with other annuals once it gets warmer. 

For example, since pansies can handle freezing temperatures, they are often planted early (as early as Mid-March for Central PA). Then, a month or so later, geraniums, vines and petunias can be planted along with them. Pansies don’t perform the best in the heat of the summer so this allows you to enjoy the peak of these plants’ beauty for a longer period of time.

What's the size of your plant

4. Size of the plant

The size of plants affects how established and ready to plant outside it really is. Obviously, seedlings won’t last very long during typical spring weather.

Many greenhouse and garden centers sell plants in varying sizes of pots and packs, so you’ll want to consider the size of the plants when determining when to plant your flowers. Some of the more popular sizes of packs and pots that you’ll find annuals for sale in:

  • 3 packs
  • 4 packs
  • 3 ½” pot
  • 4 ½” pot
  • 6” pot

Due to the variation in pot and pack sizes, the size of the plugs will vary significantly. Along with this, the plants’ strength, maturity, and hardiness is tied into how developed it’s root structure is. 

Simply put, the larger the plug and the more developed the root structure, the better the plant will handle the shock of being taken into a new environment.

However, keep in mind, that no matter how well developed the root structure is, cold temperatures can always kill plants that are not hardy enough to handle the weather.

Where are you planting your annuals

5. Where are you planting your annuals?

You can also consider where you are planting the annual. Whether it be a container, in your landscape, or in a hanging basket, each has its own requirements and advantages when started at a particular time.

When to start growing annuals in hanging baskets? 

For hanging baskets, the earlier the better! Hanging baskets look best when they are filled and overflowing with mature plants, so the earlier you can get them started, the more beautiful they will get and the longer they’ll be beautiful, provided, they are taken care of. 

A benefit of hanging baskets is that they are more mobile than plants in a pot and especially landscape. So, if a late frost happens, they can be taken inside overnight to protect blooms and plants.

When to start planting annuals in landscape? 

Planting annuals in landscape is where you want to be especially careful and the most conservative. Many plants have been wiped out by an unexpected late frost when planted in landscape. 

In some cases, buckets or clothes can be placed over plants when a late frost hits — it’s generally recommended to be better safe than sorry with landscape annuals.

When to plant annuals in pots? 

Unlike landscape annuals, planting annuals in pots does give them some mobility, so they can always be taken inside over the night if necessary. 

An important thing to consider is how full you are filling your pots. If you are planting your pots later in the season, you can fill the planters much fuller and have an almost instant great look. If you are planting annuals in pots earlier in the season, you’ll want to make sure that you are not overfilling the pots with plants, as the plants will grow considerably with the proper care.

Consider what type of protection it has

You’ll want to keep in mind the amount of protection your plant will have when planting. As mentioned earlier, you can always manually protect annuals by covering them with a bucket or a blanket, or taking them inside. 

Buildings, fences, etc. can also protect plants from harsher elements like wind that might make a cold snap more severe. Some gardeners also opt to plant their flowers in a mini greenhouse.

When is it too late to plant annuals?

Is it ever too late to plant your annuals? No! While there are certain times that are optimal, if you are swamped during the spring, you can certainly start and still grow some magnificent annuals starting in the summer. 

Keep in mind that some greenhouses may be sold out of plants you are looking to start too late, but there’s normally always something to grow, even if you are starting mid-summer!

What to do if you plant too early

If you plant too early and cold temperatures wipe out your plants, you can always come back and purchase flowers to try again. Many gardeners have been burned by surprise frosts (and for farmers, it can often lead to losing an entire crop), so it’s normal to be hit with a surprise cold snap from time to time.

Conclusion

To wrap things up, there are a number of factors that will influence the correct answer to “when is the best time to plant annuals?”

Consider the following when deciding if its safe to plant your annuals outside:

  1. What growing zonal area you are planting in annuals in
  2. Whether the annuals are hardened off
  3. The type of annual you are planting
  4. The size of the plants
  5. Where you are planting the annuals (or what you are planting them in)

Seasons and weather patterns vary, so there’s no exact date. But using this questions will guide you in knowing when the safest and best time to plant your annuals is.