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Hardening Transplants & Other May Gardening Tips
by Leonard Perry
by Leonard Perry


In extension I serve as an advisor and consultant to the greenhouse and nursery industry, primarily in Vermont but throughout the region and beyond as well.

I give presentations on my research to the industry, and to home groups. In Research, my focus is "herbaceous perennial production systems".

His website is at  Leonards zone of gardening: home with my trials, generally USDA 4a. Campus in Burlington is 5.

May 1, 2005

Hardening transplants, dividing perennials, and pruning roses are some of the many garden tips for this month.

Acclimate greenhouse-grown transplants over the course of a few weeks before setting them into the garden. Begin by placing them in a sheltered spot during the day and bringing them in at night. Then gradually increase their exposure to sun, wind, and cool temperatures. Once they're in the garden, be prepared to cover them if a late cold snap threatens.

Transplants of snapdragons can be planted outdoors now. They can tolerate below-freezing temperatures and flower best in cool weather. Plant them in groupings for the best show. You just can't have too many snapdragons!

It's time to start planting cole crops, such as broccoli, cabbage, and cauliflower. These crops should be planted about two weeks before your last frost date, in soil amended with compost. Don't be concerned if the leaves turn red or purple. It's often a sign of phosphorus deficiency due to cool soils and will go away once the soil warms.

Plant small sections of leaf lettuce every two weeks or so, rather than planting the whole bed at once. That way, you'll have tender, young leaves to harvest throughout the spring. As spring heads toward summer, choose non-bolting varieties.

Now is a good time to dig and divide late-blooming perennials, such as asters and daylilies. If left undivided, the plants become unproductive and overcrowded. Dig up the clump, and use a sharp spade to create pie-shaped wedges. Replant these divisions in a full-sun location in well-drained soil, and water often to keep soil moderately moist.

Cut back dead rose canes to healthy tissue and remove any spindly branches. Make a slanted cut that angles up toward an outward-facing bud, starting 1/4 inch above the bud. This directs the new canes to grow outward which improves air circulation in the middle of the plant to reduce risk of diseases like black spot.

Check apple, cherry, and other fruit trees for nests of tent caterpillars. They will emerge at the same time the leaves sprout. Blast nests with a strong spray of water to destroy them, or spray BT (Bacillus thuringiensis) on them. BT will harm only the caterpillars and not other beneficial insects, birds, or humans. You'll need a pump sprayer for this job. Use the correct form of BT, and follow all label directions and precautions.

To reduce watering, incorporate water-absorbing crystals into the potting mix when you plant. It's hard to add them afterwards. Since clay pots dry out faster than plastic, use plastic pots set inside clay pots to help hold in moisture. Grouping pots together also will help reduce moisture loss.

Make a note of gaps in your spring bulb garden, and plan to plant bulbs there this fall. By choosing a variety of bulbs, from early-blooming snowdrops to late-blooming alliums (ornamental onions), you can have a colorful show for months. Note the bloom times in plant descriptions. For example, Kaufmanniana tulips bloom early, while single, late tulips wrap up the spring show.

For many more tips, check out the National Gardening Association’s regional reports (

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