March is a Key Fertilizing Month But Don’t Feed These 7 Popular Garden Plants or You'll Regret it

As soon as the sun pops back out and backyard buds start to burst into blossom, it’s so tempting to start feeding plants and boost that welcome spring growth. But while many backyard favorites like roses, bougainvillea and hibiscus love a spring feed, fertilizing some plants now will do more harm than good.

The problem with spring fertilizing is that it encourages lots of tender top growth that can be zapped by a late frost. Another issue is encouraging fast-growing vegetation, so you end up with over-long, weak stems that aren’t strong enough to hold up their flowers come summer. The juicy shoots that result from a concentrated input of nutrients can be a tempting treat for pests, too.

Also, if you fuel more growth above the ground than the root system below the soil surface can support, you'll create a stressed plant that’s dependent on you for water and nutrients, and is more susceptible to disease. And some plants simply do better in poorer soils, so don’t need fertilising at all. So, before you go a garden-crazy with those fertilizer granules, here are the plants not to feed in March.

1. Lavender

This fragrant lady is native to the Mediterranean region of Europe, where it thrives in rocky and sandy soils and a hot, dry climate. So, if you fertilize lavender, you’ll create fertile conditions it’s simply not equipped to deal with. The result? You’ll enjoy fewer flowers as your Lavandula will focus on growing leaves and stems. That fast growth means that essential oils are less concentrated too, so it won’t be as fragrant in summer, or as hardy come winter. The worst case scenario is that over-fertilizing can kill your lavender plant, so leave this lilac-spired beauty be.

2. Black-Eyed Susan

Rudbeckia is another sun-lover that does best in lean soil. By all means apply a mulch to the ground around the plant to give the soil a general boost and improve its structure for good drainage, but black-eyed Susan doesn’t need fertilizing. Give it too much food and you’ll get fewer flowers, and stem congestion can lead to powdery mildew. It’ll also become less drought-tolerant, and its ability to cope with long spells of dry weather is one of this cheery daisy’s biggest assets.

3. Bee Balm

Monarda is also a highly drought-tolerant plant that never needs feeding in a border. Like many North American natives, bee balm has evolved to survive and thrive in this climate, so don’t give it conditions it’s not used to by fertilizing. All you’ll do is fuel weak, floppy stems. Other natives that won’t appreciate a feed are purple coneflower (Echinacea), Joe Pye weed (Eutrochium), woodland sedum (Sedum ternatum), butterfly weed (Asclepias tuberosa) and blanket flower (Gallardia).

4. Ornamental Grasses

Feed an ornamental grass and you’ll get weak, floppy growth. Most grasses grow quickly even on poor soil, so are best left to fend for themselves. It’s a common misconception that Miscanthus grasses are an exception to this rule. Although many miscanthus can grow to a considerable height, they don’t need feeding; indeed, some are considered invasive in parts of the US so certainly don’t need any help to grow! If your plant isn’t flowering, then the cause isn't likely to be poor nutrients. It's far more probably that it’s not getting enough sunshine so dig it up, divide the root clump to reinvigorate the plant, and replant a sunnier spot.

5. Warm-Season Turfs

Grass varieties often used in hot climates to stay green through scorching summers only wake up once temperatures are warm enough to support growth. Fertilize these warm-season grasses now and the dormant roots won’t absorb the nutrients: the fertilizer will be washed away which isn’t just wasteful, it leads to pollution. Common varieties include Bermuda grass (Cynodon dactylon), Zoysia, St. Augustine or buffalo grass (Stenotaphrum secundatum), and centipede grass (Eremochloa ophiuroides). Wait until these warm-season turfs turn green and are actively growing before feeding, which is typically in late April or May.

6. Sneezeweed

Drought-tolerant perennial plants like Helenium don’t benefit from fertilizer when growing in the ground. Excessive nutrients will cause rapid growth and tall, leggy stems which are likely to flop over – and sneezeweed often needs staking in the first place, so don’t make your life any more difficult! Far better to nourish poor soil with an annual mulching and, if your plant isn't flowering well, dig it up, divide the root clump and replant the sections to revitalise growth.

Another issue that excess fertilizer can cause is root burn. Heleniums grow an extensive root system but a shallow one, extending only around a foot deep. A concentration of unused fertilizer in the soil can damage the roots, making them less able to absorb water, so reducing the plant’s drought tolerance.

Other highly drought-tolerant perennials that are better off with a yearly much rather than fertilizer include catmint (Nepeta), sea holly (Eryngium), Russian sage (Perovskia atriplicifolia) and yarrow (Achillea).

7. Newly Planted Trees and Shrubs

Many folk feed freshly planted trees and shrubs in spring, but they’re not doing their new plants any favors. While plants are getting established in a new home, it’s important that they spend the first year focusing on root growth so they develop a strong underground supply network. Not only do the roots anchor the plant in the ground, the tips are covered in microscopic root hairs that act like straws to suck up moisture and nutrients. The more the roots grow, the better your plant can feed and water itself.

The general rule of thumb is not to fertilize for the first year of a tree or shrub’s life in your garden, regardless of how mature it is when planted. This forces the plant to grow a strong root system as it’ll have to search for nutrients. You likely mixed some organic matter such as compost into the soil when planting, and it’s fine to mulch in spring or fall to supply general nutrients, so your plant has everything it needs for now, as long as you water regularly and deeply.

Give a newly-planted tree or shrub more concentrated food than it needs and you won’t just stunt root growth, but fuel top growth, which means lots of stems and foliage. And any plant with more leaves than its root system can support is a stressed plant that’s susceptible to disease.

When to Mulch Instead of Fertilize

For plants like warm-season turf, you simply need to wait until the roots have woken up before fertilizing – otherwise you're just feeding your lawn weeds. For plants that are better without a concentrated dose of nutrients, it's far better to mulch rather than fertilize if you have poor soil. This simply means putting a 2–3 inch layer of organic matter on the ground surface, which will slowly rot down and provide gentle nourishment while improving the structure of your soil.