One of the best tools that you have as gardeners to help us figure out your gardening schedule is the USDA zone info. The most important data that the zone info tells us is average last frost and the lowest expected temperature for your area.

One way of determining when it's the best time to plant petunias is to use the USDA zone info for Helena.

Planting Calendar for Petunias

Frost tolerance for petunias:	Tolerant of a very small amount of frost.
When to plant:	Up to 3 weeks before last frost.

Petunias do ok in moderate cold which tells us that you can plant them slightly earlier than other frost tender plants.

The earliest that you can plant petunias in Helena is April. However, you really should wait until May if you don't want to take any chances.

The last month that you can plant petunias and expect a good harvest is probably August. If you wait any later than that and your petunias may not have a chance to fully mature. You can get started a little bit earlier by starting your petunias indoors.

Last Frost Date

On average all chance of frost has passed is on May 15 in Helena. In the coldest months of winter you should expect an average low temperature of -25°F.

It's important to remember that the actual date of last frost is just an average because it is based on the USDA zone info for Helena and it is different every year. Half of the time in Helena you get a frost after May 15 so be sure to be ready to cover your petunias in the event of a late frost.

USDA Zone Info for Helena

Here is the info for USDA Zone 4b.

Average Date of Last Frost (spring)	May 15
Average Date of First Frost (fall)	September 15
Lowest Expected Low	-25°F
Highest Expected Low	-20°F

This means that on a really cold year, the coldest it will get is -25°F. On most years you should be prepared to experience lows near -20°F.

Plants to Grow in Helena

You may be interested in your other planting guides for Helena.