It's time to hurry up and wait - a review of when to plant!

We’re all so eager to get our hands in the dirt and our plants in the ground! We got such a tease of spring back in March that all this cool, rainy (snowy?!?) weather seems out of place. With some warmer weather on the horizon though, it seems a good time to talk about…when to plant!

I thought I’d start by reviewing a few terms that gardeners refer to all the time.

Our Plant Hardiness Zone: 6a

Plant hardiness zones are determined by the US Department of Agriculture, and are based on the average annual minimum winter temperature. Our zone is 6a, meaning our average minimum temperature over the winter is between -5 and -10°F. Plants that are hardy up to zone 6a will be able to withstand our winter temperatures and thrive here as perennial plants.

Average Last Frost Date: ~ May 15

This is the average date of the last light frost in the spring. It is calculated with data from the NOAA (National Oceanic and Atmospheric Association). This date is an estimate and can vary depending on the source! After the frost date, it is relatively safe to put out your warm-weather loving plants. There is generally only about a 10% chance that a frost will happen beyond this date.

Also - frost can happen at temperatures above 32°F, especially on clear nights. Due to a variety of factors, the temperature at ground level can be 5 to 10 degrees cooler than the air above. The National Weather Service says that patchy frost can still occur at temperatures between 38 and 42°F, larger areas of frost from 33 to 37°F, and widespread frost/freeze at temperatures 32°F and below.

Using this information, when is it safe to put out our plants?

Perennials:

It is safe to plant perennials anytime the ground is not frozen! March and April temperatures are warm enough for perennial plants to take root and get established. In fact, a perennial will generally do best if it is planted in early spring or in the fall, after the hottest temperatures are behind us.

Hardy Annuals:

Some annual flowers and vegetables can withstand a light frost. Osteospermum (African Daisy), calibrachoa (Million Bells), ageratum and of course, pansies, do well even with some cooler temperatures. Cole crop vegetables also do well in cooler temperatures - broccoli, kale, cabbage, brussels sprouts. Hardy annuals are fine to be outside at this point in the year!

Tender Annuals:

Most of the plants we think of as annuals are tender annuals, and not only do they not like frost, but they generally prefer it to be warm - lows in the high 40s or 50s. They prefer warmer soil and although they may survive, they may not thrive if planted too early. So, even if our frost date comes and goes, you should make sure that the ground itself is warmed up before planting these. That usually means waiting until after we have a string of nice, warm, sunny days with moderate temperatures at night.

Many vegetables are in this category too -for example, tomatoes, pumpkins, and peppers are all native to central America and need warmer air and soil temperatures for strong growth.

For tender annuals, we usually recommend planting Mother’s Day weekend at the earliest, but if you can be patient and hold off until Memorial Day, that’s sometimes even better!

Hanging Baskets & Containers

Most of our hanging baskets and containers are tender annuals. They can be outside on warmer, sunnier days, but they need to be brought in overnight and if the weather is cold, or cool and wet and windy.

Hopefully in a week or so we’ll all feel safe putting out our tender annuals and leaving out our containers and baskets. Watch the weather and plan accordingly! In the meantime, bring tender annuals inside during these cooler days and nights and they’ll do better for you in the long run. And don’t forget that while you’re waiting for sunny, warm days, it’s actually the perfect time to plant some perennials to enjoy for years to come.

See you at the greenhouse!