Hillside Greenhouse

Weather and Climate

Covering Plants in Cold Weather

In our area, we can get freezing night-time temperatures until the end of May. The average last frost is typically around May 10th to May 15th. But this is only an average date -- it could still freeze even late in the month. We usually assume that the weather will be frost-free as of Memorial Day weekend.

Most spring plants are tender and must be protected if temperatures drop below freezing. It's best if you don't put these plants out until later in the spring, but if you decide to put them out early, you need to be prepared to cover them, or if they are in containers, you can move them into a garage or porch for the night. Plants can be protected by being covered with even a light sheet. Plastic is a poor insulator and should not be used.

Temperatures can very depending on location. If you are located in a low area where cold air settles, you can experience a frost whereas all your neighbors were above freezing.

Hardiness Zones

The idea of hardiness is mostly related to perennial plants such as trees, shrubs, and perennial flowers. Various types of plants are adapted to different climates. You can't expect to grow an orange tree in Minnesota, but on the other hand, some northern plants do not do well in hot climates.

You may often see perennial plants advertised in catalogs or shown in magazines that will not do well in our area. The good news is that there is almost always another good plant that will do the job and grow successfully in our climate.

Perennial plants are rated with plant growth zone numbers. The twin cities area is on the edge of zones 3 and 4. So any plant rated as zone 4 should grow here. Those rated as zone 3 or lower should be especially hardy and do very well.

You really don't need to spend a lot of time studying zone maps as long as you know we need plants of zone 4 or less for our area. If you are careful and are willing to do extra work to protect your plants through the winter months, you can often grow plants that are adapted to the zone 5 area.

Even though the USDA map is considered the standard, you should also be aware that some other groups publish maps of hardiness zones that don't use the same numbering system as the USDA map. The Sunset Magazine map is an example of one of these. The Sunset zones cover the western states and are usually considered the standard in those areas. Make sure the information that you are using follows a consistant numbering system for hardiness zones.

A second reference for climate is the American Horticultural Society Heat Zone Map. This is more commonly used in the southern states where high temperatures can be a problem. Even though Minnesota summers can be quite warm, we rarely experience the degree of heat and humidity that exists in the southern states.

Sometimes, within a plant species, you find some varieties that are more hardy than others. For example, some hollyhocks may be hardy only to zone 5 and others as hardy as zone 3, depending on the variety. By choosing varieties carefully, you can grow plants that may usually be considered not hardy in our area.

This can also lead to problems. Sometimes catalogs are very optimistic when listing zone numbers. If you're buying plants or seed from a catalog, it's best to check the hardiness ratings from several sources before purchasing.

Dealing with Drought

All plants need water to survive, although there are some that need less than others. If you are planting in an area where you know drought may be a problem, you should consider plants such as portulaca or begonia that are somewhat succulent and handle drought better than other species. Even though these survive with less water than some other plants, they cannot be allowed to go with absolutely no water during periods of drought.

The best way to make sure plants have adequate water is to have a ready supply available with a hose or watering can. Plants in containers may need to be watered nearly every day during hot dry weather. Plants in the ground can usually live on about 2 inches of water per week if the plants are well rooted into the soil.

Another way to deal with drought is to make sure you have good soil. Soil that is too sandy will not hold much water. The soil can be improved by adding organic matter such as peat or compost.

Working With Wet Weather

Wet weather is more of a problem than dry. At least when it is dry, you can bring water to the plants. When it's too wet there is little you can do. Very few garden plants are acclimated to wet soil.

By planning ahead, you can help the situation by adding organic matter or building raised beds to improve drainage.