How to Care for Plant Seedlings
Marie Iannotti is a life-long gardener and a veteran Master Gardener with nearly three decades of experience. She’s also an author of three gardening books, a plant photographer, public speaker, and a former Cornell Cooperative Extension Horticulture Educator. Marie’s garden writing has been featured in newspapers and magazines nationwide and she has been interviewed for Martha Stewart Radio, National Public Radio, and numerous articles.
Catherine Askia is a master gardener and member of The Spruce Gardening and Plant Care Review Board. She has over 30 years of experience as a home gardener designing, planting, and maintaining ornamental flower, vegetable, and herb gardens.
The Spruce / Margot Cavin
When you start seeds indoors, the tender seedlings are dependent on you for all their needs. This includes getting fed. Some gardeners think their seedlings will grow faster if they give them fertilizer right away. However, while those tiny plants may look helpless, they don’t need anything other than water, warmth, and light for their first few weeks. They are capable of feeding themselves up to a point. After that, it’s time to start feeding them, following a few standard guidelines.
When to Start Fertilizing Seedlings
When seedlings first poke out of the ground, they are still feeding off the food stored in the seed. The first couple of leaves that form are not leaves at all. They are called cotyledons or seed leaves, which are part of the seed or embryo of the plant. Cotyledons contain the remainder of the stored food reserves of the seed, and they keep the seedling fed until the first true leaves sprout and the plant can begin photosynthesis.
Usually, the cotyledons disappear shortly after the first true leaves form and begin photosynthesizing. It is at this point that the seedling can use a little boost of fertilizer.
Before you reach for the plant food, make sure you haven’t used a potting mix that already contains fertilizer. Some do, and some don’t. If the mix has fertilizer, you shouldn’t need to add more. For the future, because seedlings can initially feed themselves, you don’t need to use a potting mix with fertilizer for starting seed. Using a mix without fertilizer is cheaper, and more importantly, you can control how much and what type of food your seedlings get.
Seedlings tend to need a fertilizer that’s high in phosphorous. Phosphorus stimulates root development and is a component of photosynthesis. Look for a 1-2-1 N-P-K (nitrogen-phosphorus-potassium) ratio on the fertilizer label. A liquid or water-soluble fertilizer is typically the easiest and quickest way for the seedlings to access nutrients. You’ll also have a choice between organic and synthetic fertilizer, which often comes down to personal preference.
- Synthetic fertilizer: If you are using synthetic fertilizer, feed your seedlings weekly. However, it’s often wise to dilute the label’s recommendation by at least half. Tender seedlings can be easily burned by too much fertilizer. Young seedlings commonly can get away with a quarter of what the label recommends for full-grown plants.
- Organic fertilizer: There are several liquid organic fertilizers available, though they sometimes can be hard to locate. A mix of fish emulsion and kelp can also give your seedlings the nutrients they need to get started and reduces the risk of burning your seedlings. As with synthetic fertilizer, give your seedlings a dose of organic food weekly. Unless the product is labeled specifically for seedlings, dilute it by at least half the recommended dose. It’s better to give your seedlings a little food regularly than to risk burning those tender roots with too much fertilizer at once.
- Another option: Mix a granular organic fertilizer into the potting soil. Many gardeners do this when their seedlings are ready to be moved from their starter containers to larger pots. However, granular fertilizer can take a while to release nutrients and impact the plants, so adding it when you are starting your seeds is often a better option. Try to add it to the lower layer of potting mix, and don’t let it come in direct contact with the seeds. Even organic fertilizers can burn if you use too much.
Knowing When Seedlings Have Had Enough Food
How much to feed seedlings will take some experimentation. Keep an eye on how well your seedlings are filling out. Too much fertilizer can cause a flush of tender, lanky growth, which is not what you want. Ease back on the fertilizer if this is the case. At this point in a seedling’s development, you should be more interested in growing a healthy root system than sending up a lot of green leaves.
Moreover, each plant—even those of the same species—will react a little differently to fertilizer. But in time you should get a feel for how much food it takes to keep your seedlings robust while they build up the strength to be moved outdoors into the garden.
Watch Now: 6 Mistakes to Avoid When Growing Seeds Indoors
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Do I Weed & Feed or Plant Seed First?
You can sow grass seeds now and kill weeds later with a post-emergent herbicide or kill weeds now with a pre-emergent herbicide and plant the seeds later. Weed-and-feed fertilizers are specially formulated combinations of turf fertilizer and herbicides that you apply either before you plant grass seeds or on established lawns.
You apply pre-emergent herbicides before weed seeds germinate, typically in the spring. Pre-emergent herbicides do not prevent weed seeds from germinating; they suppress the development of weed roots as they germinate. They’re usually effective for two weeks to three months, depending on the formulation, and you have to water the lawn after applying for the herbicide for it to be effective. There are pre-emergent herbicides to kill both broadleaf weeds and weedy grasses. You can apply them before you sow your grass seed. If you apply a pre-emergent herbicide that kills weedy grasses, you have to delay sowing your lawn seed.
Post emergent herbicides kill weeds after they appear. Some post-emergent, systemic herbicides that you can apply directly on lawns only kill weedy grasses, while others only kill weeds with broadleaf weeds. Contact herbicide such as those including the active ingredient glyphosate kills on contact. To use one of those on a lawn without killing the grass you have to daub it on individual weeds.
Pre-Emergent Weed-and-Feed Fertilizer
Fertilizers containing pre-emergent herbicides selectively prevent certain kinds of weeds from finishing their germination cycle. There is no point applying this type of weed-and-feed mix after weeds are growing on your lawn. You have to apply it early in the growing season before weeds appear. Make sure the pre-emergent herbicide in the fertilizer kills the kind of weeds that have plagued your lawn in the past. You might apply a starter fertilizer containing a pre-emergent herbicide before you sow your lawn seeds.
Post-Emergent Weed-and-Feed Fertilizer
Post-emergent weed-and-feed formulations kill selective weeds that are already growing in your lawn. Make sure that the herbicide in the formulation you buy kills the type of weeds that are growing in your lawn. Most weeds make their appearance in the spring, the best time to apply weed-and-feed fertilizer.
Weed & Feed Around Plants
A lush, weed-free lawn adds curb appeal to your home. Weed and feed products provide fertilizer for the grass, and herbicides, such as 2,4-D, to control weeds. These herbicides usually enter the weeds through the roots and kill the weeds in a few days. Those same chemicals that keep weeds out of your lawn can find their way to trees and other desirable plants in your yard.
Weed and Feed Herbicides
According to the University of California Statewide Integrated Pest Management Program, many weed and feed products contain selective post-emergent herbicides, such as mecoprop-p and 2,4-D. These herbicides kill weeds after they germinate, so you should use them only if you see weeds in your lawn. Some starter fertilizers contain a pre-emergent herbicide, which prevents seeds from germinating and controls weeds before they sprout. If you use a lawn service, it’s possible employees use herbicides, such as the pre-emergent fertilizer siduron, that are only available to professionals.
Herbicide Damage to Ornamental Plants
Ornamental plants, including flowers, shrubs and trees, may be sensitive to the herbicides in weed and feed products. For example, 2,4-D and MCPP can cause leaf cupping, stunted growth and may kill a dogwood (Cornus florida), which the University of Florida Extension says grows in U.S. Department of Agriculture plant hardiness zones 5a through 9a, or Western hackberry (Celtis laevigata var. reticulata), which grows in USDA zones 3 through 9.
These herbicides can also kill or damage turf grasses, including St. Augustine grass (Stenotaphrum secundatum), which is hardy in USDA zones 8 through 10, and carpet grass (Axonopus affinis), which grows in USDA zones 8 through 9. Symptoms can appear within a week after spring application, or in the spring following fall application. Herbicides can also cause temporary discoloration in tolerant grasses.
Weed and Feed Application
Each weed and feed product has specific instructions and warnings on the label. You can spread 2.9 pounds of MCPP- and 2,4-D-based granules per 1,000 square feet of lawn, for example, when weeds are actively growing and temperatures stay between 60 and 90 degrees Fahrenheit.
To keep the product away from desired flowers, trees and fruit and vegetable plants, avoid applying when it’s windy or if rain is predicted within 24 hours, and do not apply inside a tree’s drip line. The granules will stick better to recently watered grass. You can repeat the weed and feed application after 30 days, but do not apply more than twice per year.
People and pets should stay away from treatment areas during weed and feed application, and until the product’s dust settles. Wear protective clothing including gloves, long sleeves and long pants when you use weed and feed products. The label may also tell you to use other safety equipment. Before you use a weed and feed product, make sure the fertilizer in the product is suitable for your turf grass.
For example, Bermuda grass (Cynodon spp.) grows in USDA zones 7 through 10, and benefits from 2.9 pounds per 1,000 square feet of a 2,4-D and MCPP weed and feed product that contains 28-0-3 fertilizer in the spring and fall. Bermuda grass can become invasive, so a barrier may be necessary to keep it out of flower beds or other garden areas.
- University of California Statewide Integrated Pest Management Program: Weed Management in Lawns
- University of Florida IFAS Extension: Cornus Florida: Flowering Dogwood
Judith Evans has been writing professionally since 2009, specializing in gardening and fitness articles. An avid gardener, Evans has a Bachelor of Arts in political science from the University of New Hampshire, a Juris Doctor from Vermont Law School, and a personal trainer certificate from American Fitness Professionals and Associates.