Before planting it is a good idea to find your USDA Zone info to estimate your garden schedule. The zone info helps tell us things like average date of last frost and lowest expected temperature for your area.

In Boulder you can determine when it's possible to plant romaine lettuce by referring to the USDA zone info.

Planting Calendar for Romaine lettuce

Frost tolerance for romaine lettuce:	Very tolerant of frost.
When to plant:	Up to 7 weeks before last frost.

You can plant romaine lettuce earlier in the year because they are one of those plants that can tolerate quite a bit of cold weather.

The earliest that you can plant romaine lettuce in Boulder is February. However, you really should wait until March if you don't want to take any chances.

The last month that you can plant romaine lettuce and expect a good harvest is probably September. You probably don't want to wait any later than that or else your romaine lettuce may not have a chance to really do well. If you are starting your romaine lettuce indoors then you might be able to get away with starting them a little bit earlier.

Last Frost Date

On average all chance of frost has passed is on April 15 in Boulder. It might get as low as -15°F during the coldest months of winter.

Remember that the actual date of last frost is an average because it is based on the USDA zone info for Boulder and it will vary from year to year. Half of the time in Boulder last frost occurs after April 15 so make sure that you are prepared to cover your romaine lettuce if you have one of those late frosts.

USDA Zone Info for Boulder

Here is the info for USDA Zone 5b.

Average Date of Last Frost (spring)	April 15
Average Date of First Frost (fall)	October 15
Lowest Expected Low	-15°F
Highest Expected Low	-10°F

This means that on a really cold year, the coldest it will get is -15°F. On most years you should be prepared to experience lows near -10°F.

Plants to Grow in Boulder

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