Most flowers should be planted after your region's last frost date. Planting flowers in spring is the most popular time, but perennials do fine if planted in early fall in the North and late fall in the South.
The best month to plant flowers in spring is really down to the USDA hardiness zone you live in and the likely last frost date. This is likely to be from around March, but if you live in a northern zone, be sure to plant frost-tolerant perennials until you are happy that the risk to more tender flowers has passed.
So, how hot IS too hot for plants? The general answer is around 90 degrees F, with some exceptions to the rule. This means that when temps rise above 90 and remain there for a lengthy spell: Leaves wilt.
Spring blooming bulbs can be planted any time as long as the ground isn't frozen. Shrubs and trees can be planted, however, earlier in fall is better as it allows for best root establishment before winter. Cold tolerant perennials can be planted, but they will be more winter hardy if planted earlier in fall.
It's best to wait until the last freeze of the season has passed before planting anything outside. Typically for the DMV the last freeze of the season occurs between the last week in March and the first week in April. A good rule of thumb is to wait until Mother's Day to plant anything outdoors.
I did some research and found out that houseplants need to be taken indoors before overnight temperatures dip below 45 degrees. Most tropical plants suffer harm from temperatures below 40 degrees.
Generally, the ideal temperatures for vegetable plant growth are between 40° and 85°F. At warmer temperatures the plant's growth will increase, but this growth may not be sound structural growth. At lower temperatures the plant's growth will slow down or stop altogether.
Plants and Planting
Pansy roots will not function when soil temperatures are below 45°F. When soil warms to about 65°F (about two to three weeks before frost-free date), cool tolerant plants can be planted, such as nemesia, diascia, snapdragons, alyssum, osteospermum, mimulus, lobelia and petunias.
April is a great time to plant many types of flowers. Some of the best choices include pansies, petunias, snapdragons, and lilies. Pansies are ideal for cooler climates as they can tolerate light frost and bloom in bright colors like yellow, purple, or white.
Generally speaking, spring-flowering bulbs should be planted in the fall (a few weeks before first frost), while summer-flowering bulbs should be planted in the spring (a few weeks after last frost).
At or near the bloom stage, the critical temperature is the same for almost all fruits and flowers. Freezing temperatures of 28 degrees Fahrenheit will result in about a 10 percent loss and 24 F in a 90 percent loss.
For most flower types, optimal storage temperatures range between 33°F - 37°F while cold-sensitive blossoms and tropical flowers should be maintained at temperatures above 50°F.
The rule of thumb has always been to plant in spring and fall when the weather is cooler. But a rising temperature doesn't mean you can't continue to fill the empty spaces in your garden with plants. Sure, there are some things you shouldn't plant in summer, such as a bare-root, newly dug or newly divided plant.
Gardening in the fall provides a longer period with more 'good' days, as compared to the often tumultuous spring season. Plus, if you plant in fall you'll end up ahead of the game and won't have to rush to get everything done after winter. In fact, a good strategy is to plant perennials when you plant your fall bulbs.
Flowers that come back up from their roots every year in the springtime are called “perennial” flowers. Some of the best perennial flowers to grow in a garden are peonies, daylilies, coneflowers, and hollyhocks.
Turn the soil over to a depth of at least 12 inches. Add 2-3 inches of compost and turn it into the bed. Either cover the bed with a thick (3-4") layer of mulch or use a weed and feed to help keep weed seeds from germinating. Top dress with another layer of compost to keep down weeds and preserve moisture.
Some of the best flowers to plant in May can be grown directly from seed, including poppies, lantana, and sunflowers. Other choices do best when purchased as established plants, such as marigolds or hibiscus plants. However, some flowers planted during this month come back year after year, such as lavender and catmint.
Perennial flowers to plant in April
Plant hardy perennials, such as daylilies and delphiniums. Later in the month, when the weather settles, set out transplants of: Forget-me-nots (Myosotis spp.) Foxglove (Digitalis spp.)
Perennials to plant in March
Summer and fall bloomers can be lifted and divided now. Daylilies, chrysanthemums, hostas, daisies, and coreopsis are easy to divide and transplant when still semi-dormant.
The Basics of Planting Annual Flowers
Tender annuals — Tender annuals thrive in warm soil and warm air temperatures. They should only be planted once temperatures reach and stay above 55°F. They can be severely damaged or die if they are exposed to frost or temperatures below 32°F.
With that in mind, experts generally recommend flowers and other houseplants be brought inside or otherwise protected before the thermometer dips below 45 degrees Fahrenheit. For warmer-weather and tropical plants, that threshold is a bit higher, at around the 50-degree mark.
If you're wondering at what temperature threshold you should be bringing your plants, there's a short answer: when nighttime temperatures reach 45 degrees (F), it's time to bring your plants indoors.
When Should You Cover Plants? Cover your plants at night and remove them during the day when the temperatures rise above 32 degrees F, so that the soil can warm up again. Some outdoor plants won't survive the harsh conditions of winter, bring them inside and use these tips for caring for them through winter.
For many plants, temperatures below 50°F can cause problems. Adjust thermostats to cater to your comfort, but remember your plants need some consideration. Avoid placing plants near cold drafts or heat sources. Keep plants several inches away from exterior windows.
Overall even flowers can tolerate light freezes (low 30s to upper 20s °F) and even a little snow. Record cold temperatures (below mid to upper 20s) will damage or destroy many of the flowers of early blooming varieties. The foliage of fully emerged spring-flowering bulbs like tulips and daffodils can also be damaged.