May is the busiest month for gardeners in Southeast Wisconsin. Soil temperatures climb past 60°F by mid-month, the last frost typically passes between May 10 and 20, and almost every job that’s been waiting for warm weather lines up at once. If you only have a few weekends to make things happen, this is the month to spend them outside. Here are six tasks worth doing in your Zone 5b garden right now.
1. Plant Warm-Season Vegetables and Annuals
Once nighttime temps stay reliably above 50°F and your soil thermometer reads 60°F or warmer at four inches deep, the warm-season crops can go in. Tomatoes, peppers, squash, cucumbers, beans, basil — they all want this kind of weather. Same goes for the heat-loving annuals: marigolds, petunias, zinnias, cosmos, and sunflowers will take off in May soil.
One small thing that makes a big difference: bury tomato transplants deep, all the way up to the first true leaves. Roots will form along the buried stem and you’ll end up with a stronger, more drought-resistant plant.
Harden off before transplanting:
If you started seeds indoors, give them a week of gradual outdoor exposure before they go in the ground. A few hours in shade on day one, longer and sunnier each day after. Skipping this step is the most common reason transplants stall out or get sunburned in their first week outside.
Watch the forecast through May 20. A late cold snap is uncommon but not unheard of in Southeast Wisconsin, and a single 32°F night can wipe out tender seedlings. Keep a few row covers or old sheets handy.
2. Divide and Multiply Your Perennials
Spring is the right time to divide most summer- and fall-blooming perennials before they put on serious growth. Hostas, daylilies, sedums, asters, coneflowers, and ornamental grasses all respond well to division when the new growth is just a few inches tall. You’ll get healthier, more vigorous plants, you’ll keep mature clumps from outgrowing their spot, and you’ll come away with free starts to fill bare ground elsewhere or share with a neighbor.
The process is straightforward. Dig the entire clump, lift it out of the ground, and split it with a sharp spade or two garden forks back-to-back. Replant the divisions promptly at the same depth they were before, water deeply, and mulch around the new plantings to hold moisture and reduce stress while the roots re-establish.
Skip dividing fall-blooming perennials in May. Mums, asters, and Japanese anemones do better with a fall division after they’ve finished flowering.
3. Feed What You’ve Already Planted
The burst of spring growth depletes soil nutrients fast. Established perennials, shrubs, and trees benefit from a balanced slow-release fertilizer applied in early to mid-May. The slow-release part matters: it feeds the plant gradually over several weeks instead of all at once, which means less leaching and steadier growth.
Acid-loving plants like azaleas, rhododendrons, blueberries, and hydrangeas have their own fertilizer formulations. Don’t substitute a general-purpose product for these — the soil chemistry is different and a regular fertilizer can actually push the pH the wrong direction over time.
Time it with the rain:
Apply fertilizer right before a rain if you can. The water carries the nutrients into the root zone evenly. If rain isn’t in the forecast, water the area thoroughly after applying. Either way, follow the rate on the label. More isn’t better, and over-fertilizing can burn roots and push too much soft growth that struggles in summer heat.
4. Prune Spring-Flowering Shrubs Right After They Finish
Lilacs, forsythia, viburnum, mock orange, and other spring bloomers set next year’s flower buds during the summer following bloom. That means the pruning window is narrow: as soon as the flowers fade, before the plant invests energy in next year’s buds. Wait too long and you’ll cut off your own flowers for the following spring.
Focus on three things when you prune:
- Shape: Reduce overall height and width if the shrub is outgrowing its space, but don’t take more than a third of the plant in one season.
- Thin: Cut a few of the oldest, thickest stems all the way to the ground. This opens up the interior to light and air, and encourages new vigorous growth from the base.
- Clean up: Remove anything dead, broken, or rubbing against another branch.
Summer-flowering shrubs work the opposite way. Rose of Sharon, smooth hydrangea (the Annabelle types), and panicle hydrangea bloom on new wood, so they get pruned in late winter or very early spring before growth starts. Don’t touch those now.
5. Get Ahead of Pests Before They Take Over
As the weather warms, the regulars start showing up: aphids on new growth, slugs on hostas, beetles on roses and ornamental cherries. The best time to deal with pests is before the population explodes, and May is when you can still get ahead of most of them with light intervention.
A few low-effort things that work:
- Encourage beneficials: Ladybugs, lacewings, and predatory wasps eat huge numbers of aphids and small caterpillars. Plant some yarrow, dill, or sweet alyssum nearby to attract them.
- Hand-pick what you can: A bucket of soapy water and ten minutes after dinner handles a surprising amount of beetle and slug pressure.
- Use row covers on young vegetables: A lightweight floating cover keeps cabbage moths off broccoli and carrot rust flies off carrots until plants are big enough to handle some damage.
- Keep beds weed-free: Weeds shelter pests and pull moisture away from the plants you actually want. A weekly walk-through with a hoe pays off through July.
Skip the heavy chemicals if you can. Most early-season pest pressure responds to physical removal, beneficial insects, and good garden hygiene. Save the targeted treatments for situations where those approaches aren’t keeping up.
6. Install Stakes, Cages, and Trellises Now
This is the tip most people learn the hard way. Plant supports are easier to install when plants are small. Try to add them in July when tomatoes are sprawling and peonies are flopping over from the first heavy rain, and you’ll be working around brittle stems, tangled foliage, and roots that resent being disturbed.
Set tomato cages or stakes when you transplant. Drive trellis posts for pole beans, peas, and cucumbers before the seedlings emerge. For heavy-blooming perennials like peonies, tall phlox, and dahlias, peony hoops or grow-through grids work best when slipped over the crown while the new shoots are still under six inches.
The supports disappear into the foliage as plants fill out, and the structure is already there when wind, rain, or sheer flower weight would otherwise lay everything down.
FAQ
When is the last frost in Southeast Wisconsin?
The average last frost in Zone 5b runs from about May 10 to May 20, with some variation year to year. A late frost can still hit through Memorial Day weekend in cooler years, so watch the forecast through late May if you’ve already planted tender annuals or warm-season vegetables. Row covers or old sheets draped over plants overnight handle most short cold snaps.
Should I cut the leaves off spring bulbs after they bloom?
No. Leave the foliage intact until it yellows and dies back naturally, which takes about 6 to 8 weeks after blooming. The leaves are still photosynthesizing and feeding the bulb for next year’s flower. You can snip the spent flower stems for a tidier look, but don’t touch the leaves until they’ve gone fully yellow.
How do I know when my soil is warm enough for tomatoes?
A simple soil thermometer pushed about four inches into the bed gives you the answer. Read it in the morning for a few days in a row. Tomatoes want consistent readings of 60°F or above before they’ll really start growing. In Southeast Wisconsin, that’s typically around May 15 to 20, depending on the year. Planting too early just means transplants that sit and sulk for two weeks before doing anything.
Stop By and We’ll Get You Set Up
Stop by Heritage Hill Nursery in Cedarburg this week for warm-season transplants, perennials worth dividing, the right fertilizer for what you’re growing, and any of the supports and tools you need to make this month count. Our crew is happy to walk through your specific yard and pick out exactly what fits. Always good to talk gardens.
50% Off Fountains & Fountainettes
All May at our Cedarburg garden center. In-store only. While supplies last.