One of the first steps you take when you are estimating your gardening schedule is find your 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.

The earliest that you can plant tree onions in Zone 4a should be figured out by looking at the USDA zone info.

Planting Calendar for Tree onions

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

Tree onions tend to make it in mild cold which tells us that you can plant them a bit earlier in the year than more sensitive plants.

The earliest that you can plant tree onions in Zone 4a 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 tree onions and expect a good harvest is probably August. You probably don't want to wait any later than that or else your tree onions may not have a chance to grow to maturity. You can get started a couple of weeks earlier by starting your tree onions indoors.

Last Frost Date

On average the last frost has passed is on May 15 in Zone 4a. In the coldest months of winter you should expect an average low temperature of -30°F.

Since the USDA zone info for Zone 4a may not be accurate from year to year the actual date of last frost can change quite a bit from year to year. Since half of the time in Zone 4a it frosts late in the year after May 15 be ready to cover your tree onions in the event of a surprise late frost.

USDA Zone Info for Zone 4a

Here is the info for USDA Zone 4a.

Average Date of Last Frost (spring)	May 15
Average Date of First Frost (fall)	September 15
Lowest Expected Low	-30°F
Highest Expected Low	-25°F

This means that on a really cold year, the coldest it will get is -30°F. On most years you should be prepared to experience lows near -25°F.

Plants to Grow in Zone 4a

You may be interested in your other planting guides for Zone 4a.